refactor(gateway): migrate Discord adapter to bundled plugin (full Teams parity)

First migration of an existing built-in platform adapter to the plugin
system established by IRC / Teams / LINE / Google Chat. Closes #24325;
advances the umbrella refactor in #3823.

Matches Teams' shape exactly — adapter under ``plugins/platforms/discord/``
with the standard ``__init__.py`` / ``adapter.py`` / ``plugin.yaml``
shell, ``register(ctx)`` entry point, **no back-compat shim** at the old
import path, and full parity for the four hooks Teams uses plus the
``apply_yaml_config_fn`` hook that landed in #25443 (the Discord plugin
is the first consumer of that hook):

* ``standalone_sender_fn`` — out-of-process cron delivery via REST API
* ``setup_fn`` — interactive ``hermes setup gateway`` wizard
* ``apply_yaml_config_fn`` — translate ``config.yaml`` ``discord:`` keys
  into ``DISCORD_*`` env vars (replaces the hardcoded block in
  ``gateway/config.py``)
* ``is_connected`` — declares connection state from ``DISCORD_BOT_TOKEN``
* ``check_fn`` — lazy-installs ``discord.py`` on demand
* plus ``allowed_users_env``, ``allow_all_env``, ``cron_deliver_env_var``,
  ``max_message_length``, ``emoji``, ``required_env``, ``install_hint``

* ``gateway/platforms/discord.py`` (5,101 LOC) →
  ``plugins/platforms/discord/adapter.py`` (git rename, R090).
* New ``plugins/platforms/discord/{__init__.py, plugin.yaml}`` with
  ``requires_env`` / ``optional_env`` declarations.
* Append ``register(ctx)`` block + new hook implementations
  (``_standalone_send``, ``interactive_setup``, ``_apply_yaml_config``,
  ``_clean_discord_user_ids``, ``_is_connected``, ``_build_adapter``,
  plus helpers ``_DISCORD_CHANNEL_TYPE_PROBE_CACHE`` etc.) to the
  adapter.

* Replace the ``Platform.DISCORD elif`` branch in
  ``GatewayRunner._create_adapter()`` (−9 LOC) with a generic post-creation
  hook (+6 LOC) in the registry path: any plugin adapter that declares a
  ``gateway_runner`` attribute now gets it auto-injected. Webhook's
  built-in branch is unchanged (it doesn't go through the registry path).

* Move ``_send_discord`` (190 LOC) and helpers
  (``_DISCORD_CHANNEL_TYPE_PROBE_CACHE``, ``_remember_channel_is_forum``,
  ``_probe_is_forum_cached``, ``_derive_forum_thread_name``) from
  ``tools/send_message_tool.py`` into the plugin as ``_standalone_send``.
* Wire via ``standalone_sender_fn=_standalone_send`` (Teams pattern; same
  gap fixed in #21804 for other plugin platforms).
* Replace the Discord ``elif`` in ``tools/send_message_tool.py``
  ``_send_to_platform`` with a 10-line registry-hook dispatch.
* Drop the ``DiscordAdapter`` import and the
  ``Platform.DISCORD: DiscordAdapter.MAX_MESSAGE_LENGTH`` ``_MAX_LENGTHS``
  entry — the registry's ``max_message_length=2000`` covers it.

* Move ``_setup_discord`` and ``_clean_discord_user_ids`` (68 LOC) from
  ``hermes_cli/setup.py`` into the plugin as ``interactive_setup``.
* Wire via ``setup_fn=interactive_setup``.  CLI helpers (``prompt``,
  ``print_info``, etc.) are lazy-imported so the plugin's module-load
  surface stays minimal.
* Remove ``"discord": _s._setup_discord`` from
  ``hermes_cli/gateway.py::_builtin_setup_fn``.
* Remove the entire 32-line ``_PLATFORMS["discord"]`` static dict entry —
  Discord's setup metadata is now discovered dynamically via
  ``_all_platforms()`` from the registry entry.

* Move the 59-line ``discord_cfg`` YAML→env bridge from
  ``gateway/config.py::load_gateway_config()`` into the plugin as
  ``_apply_yaml_config``.  Covers ``require_mention``,
  ``thread_require_mention``, ``free_response_channels``, ``auto_thread``,
  ``reactions``, ``ignored_channels``, ``allowed_channels``,
  ``no_thread_channels``, ``allow_mentions.{everyone,roles,users,
  replied_user}``, and ``reply_to_mode`` (including the YAML 1.1
  ``off``-as-False coercion and the ``extra.reply_to_mode`` fallback).
* Wire via ``apply_yaml_config_fn=_apply_yaml_config``.
* The hook runs BEFORE ``_apply_env_overrides`` and after the generic
  shared-key loop, exactly as documented in
  ``website/docs/developer-guide/adding-platform-adapters.md``.
* Behavior is preserved exactly — every assignment still uses
  ``not os.getenv(...)`` guards so env vars take precedence over YAML.

All 78 references to the old import path are rewritten — no back-compat
shim:

* 51 ``from gateway.platforms.discord import X`` →
  ``from plugins.platforms.discord.adapter import X``
* 5 ``import gateway.platforms.discord as discord_platform`` →
  ``import plugins.platforms.discord.adapter as discord_platform``
* 1 ``from gateway.platforms import discord as discord_mod`` →
  ``from plugins.platforms.discord import adapter as discord_mod``
* 21 ``mock.patch("gateway.platforms.discord.X")`` strings →
  ``mock.patch("plugins.platforms.discord.adapter.X")``
* 1 docstring reference in ``hermes_cli/commands.py``
* 1 import in ``tools/send_message_tool.py`` (now removed entirely)

The import-safety test in ``tests/gateway/test_discord_imports.py`` is
updated to purge the new canonical module name from ``sys.modules``.

**38 files changed, +621 / −473** — net positive due to the YAML hook
implementation (89 new LOC in the plugin trading for 59 deleted in core),
but every line moved has a clear plugin home now.  The git rename is
detected at R090 because the adapter gained ~340 LOC of moved-in hook
implementations (``_standalone_send`` + ``interactive_setup`` +
``_apply_yaml_config`` + helpers).

* All 568 Discord-specific tests pass across 25 ``test_discord_*.py``
  files plus voice/send/text-batching/reload-skills/stream-consumer/
  integration tests.
* All 147 tests in the YAML-touching subset
  (``test_discord_reply_mode``, ``test_discord_free_response``,
  ``test_discord_allowed_channels``, ``test_discord_allowed_mentions``,
  ``test_discord_channel_controls``, ``test_discord_reactions``,
  ``test_discord_thread_persistence``, ``test_runtime_footer``) pass —
  this is the strongest signal that the YAML→env hook behaves
  identically to the legacy block.
* Broader gateway/cron/integration sweep (1297 tests) introduces zero
  new failures vs ``main``.  Pre-existing failures in
  ``tests/gateway/test_tts_media_routing.py`` and
  ``tests/e2e/test_platform_commands.py`` reproduce identically on the
  unchanged ``main`` revision.
* Plugin discovery sanity check confirms Discord registers alongside the
  other four platform plugins:

    Registered platforms: ['discord', 'google_chat', 'irc', 'line', 'teams']

These Discord-shaped tendrils in core were **deliberately not moved** —
they are generic platform-registry concerns affecting every platform,
not Discord-specific:

* ``gateway/config.py:1205`` ``DISCORD_BOT_TOKEN → config.token`` env
  enablement — same shape Telegram has.  The existing
  ``env_enablement_fn`` registry hook only seeds ``extra``, not
  ``.token``, so it can't replace this without an adapter refactor to
  read from ``extra["bot_token"]``.
* ``gateway/run.py`` voice-mode hooks
  (``self.adapters.get(Platform.DISCORD)`` for
  ``start_voice_mode``/``stop_voice_mode``), role-based auth,
  ``DISCORD_ALLOW_BOTS`` branch in ``_is_user_authorized``,
  ``_UPDATE_ALLOWED_PLATFORMS`` frozenset, and the per-platform
  allowlist maps — generic platform-registry concerns.
* ``Platform.DISCORD`` enum literal — stable identifier used as dict
  keys throughout the codebase; removing it is a separate refactor with
  no real benefit.
* ``tools/discord_tool.py`` and ``tools/environments/local.py`` —
  first-class agent tools and env-passthrough config, neither is the
  gateway adapter.

Each of these is worth its own scoping issue when the time comes.

tests/gateway/test_voice_command.py

@@ -511,7 +511,7 @@ class TestDiscordPlayTtsSkip:
     """Discord adapter skips play_tts when bot is in a voice channel."""
 
     def _make_discord_adapter(self):
-        from gateway.platforms.discord import DiscordAdapter
+        from plugins.platforms.discord.adapter import DiscordAdapter
         from gateway.config import Platform, PlatformConfig
         config = PlatformConfig(enabled=True, extra={})
         config.token = "fake-token"
@@ -599,7 +599,7 @@ class TestVoiceReceiver:
     """Test VoiceReceiver silence detection, SSRC mapping, and lifecycle."""
 
     def _make_receiver(self):
-        from gateway.platforms.discord import VoiceReceiver
+        from plugins.platforms.discord.adapter import VoiceReceiver
         mock_vc = MagicMock()
         mock_vc._connection.secret_key = [0] * 32
         mock_vc._connection.dave_session = None
@@ -1066,7 +1066,7 @@ class TestDiscordVoiceChannelMethods:
     """Test DiscordAdapter voice channel methods (join, leave, play, etc.)."""
 
     def _make_adapter(self):
-        from gateway.platforms.discord import DiscordAdapter
+        from plugins.platforms.discord.adapter import DiscordAdapter
         from gateway.config import Platform, PlatformConfig
         config = PlatformConfig(enabled=True, extra={})
         config.token = "fake-token"
@@ -1208,7 +1208,7 @@ class TestDiscordVoiceChannelMethods:
 
         pcm_data = b"\x00" * 96000
 
-        with patch("gateway.platforms.discord.VoiceReceiver.pcm_to_wav"), \
+        with patch("plugins.platforms.discord.adapter.VoiceReceiver.pcm_to_wav"), \
              patch("tools.transcription_tools.transcribe_audio",
                    return_value={"success": True, "transcript": "Hello"}), \
              patch("tools.voice_mode.is_whisper_hallucination", return_value=False):
@@ -1223,7 +1223,7 @@ class TestDiscordVoiceChannelMethods:
         callback = AsyncMock()
         adapter._voice_input_callback = callback
 
-        with patch("gateway.platforms.discord.VoiceReceiver.pcm_to_wav"), \
+        with patch("plugins.platforms.discord.adapter.VoiceReceiver.pcm_to_wav"), \
              patch("tools.transcription_tools.transcribe_audio",
                    return_value={"success": True, "transcript": "Thank you."}), \
              patch("tools.voice_mode.is_whisper_hallucination", return_value=True):
@@ -1238,7 +1238,7 @@ class TestDiscordVoiceChannelMethods:
         callback = AsyncMock()
         adapter._voice_input_callback = callback
 
-        with patch("gateway.platforms.discord.VoiceReceiver.pcm_to_wav"), \
+        with patch("plugins.platforms.discord.adapter.VoiceReceiver.pcm_to_wav"), \
              patch("tools.transcription_tools.transcribe_audio",
                    return_value={"success": False, "error": "API error"}):
             await adapter._process_voice_input(111, 42, b"\x00" * 96000)
@@ -1251,7 +1251,7 @@ class TestDiscordVoiceChannelMethods:
         adapter = self._make_adapter()
         adapter._voice_input_callback = AsyncMock()
 
-        with patch("gateway.platforms.discord.VoiceReceiver.pcm_to_wav",
+        with patch("plugins.platforms.discord.adapter.VoiceReceiver.pcm_to_wav",
                    side_effect=RuntimeError("ffmpeg not found")):
             await adapter._process_voice_input(111, 42, b"\x00" * 96000)
         # Should not raise
@@ -1269,7 +1269,7 @@ class TestVoiceReceiverThreadSafety:
     """Verify that VoiceReceiver buffer access is protected by lock."""
 
     def _make_receiver(self):
-        from gateway.platforms.discord import VoiceReceiver
+        from plugins.platforms.discord.adapter import VoiceReceiver
         mock_vc = MagicMock()
         mock_vc._connection.secret_key = [0] * 32
         mock_vc._connection.dave_session = None
@@ -1282,7 +1282,7 @@ class TestVoiceReceiverThreadSafety:
     def test_check_silence_holds_lock(self):
         """check_silence must hold lock while iterating buffers."""
         import ast, inspect, textwrap
-        from gateway.platforms.discord import VoiceReceiver
+        from plugins.platforms.discord.adapter import VoiceReceiver
         source = textwrap.dedent(inspect.getsource(VoiceReceiver.check_silence))
         tree = ast.parse(source)
         # Find 'with self._lock:' that contains buffer iteration
@@ -1303,7 +1303,7 @@ class TestVoiceReceiverThreadSafety:
     def test_on_packet_buffer_write_holds_lock(self):
         """_on_packet must hold lock when writing to buffers."""
         import ast, inspect, textwrap
-        from gateway.platforms.discord import VoiceReceiver
+        from plugins.platforms.discord.adapter import VoiceReceiver
         source = textwrap.dedent(inspect.getsource(VoiceReceiver._on_packet))
         tree = ast.parse(source)
         # Find 'with self._lock:' that contains buffer extend
@@ -1670,7 +1670,7 @@ class TestStopAcquiresLock:
 
     @staticmethod
     def _make_receiver():
-        from gateway.platforms.discord import VoiceReceiver
+        from plugins.platforms.discord.adapter import VoiceReceiver
         vc = MagicMock()
         vc._connection.secret_key = [0] * 32
         vc._connection.dave_session = None
@@ -1772,7 +1772,7 @@ class TestPacketDebugCounterIsInstanceLevel:
 
     @staticmethod
     def _make_receiver():
-        from gateway.platforms.discord import VoiceReceiver
+        from plugins.platforms.discord.adapter import VoiceReceiver
         vc = MagicMock()
         vc._connection.secret_key = [0] * 32
         vc._connection.dave_session = None
@@ -1805,7 +1805,7 @@ class TestPlayInVoiceChannelUsesRunningLoop:
     def test_source_uses_get_running_loop(self):
         """The method source code calls get_running_loop, not get_event_loop."""
         import inspect
-        from gateway.platforms.discord import DiscordAdapter
+        from plugins.platforms.discord.adapter import DiscordAdapter
         source = inspect.getsource(DiscordAdapter.play_in_voice_channel)
         assert "get_running_loop" in source, \
             "play_in_voice_channel should use asyncio.get_running_loop()"
@@ -1849,7 +1849,7 @@ class TestVoiceTimeoutCleansRunnerState:
 
     @staticmethod
     def _make_discord_adapter():
-        from gateway.platforms.discord import DiscordAdapter
+        from plugins.platforms.discord.adapter import DiscordAdapter
         from gateway.config import PlatformConfig, Platform
         config = PlatformConfig(enabled=True, extra={})
         config.token = "fake-token"
@@ -1940,7 +1940,7 @@ class TestPlaybackTimeout:
 
     @staticmethod
     def _make_discord_adapter():
-        from gateway.platforms.discord import DiscordAdapter
+        from plugins.platforms.discord.adapter import DiscordAdapter
         from gateway.config import PlatformConfig, Platform
         config = PlatformConfig(enabled=True, extra={})
         config.token = "fake-token"
@@ -1964,7 +1964,7 @@ class TestPlaybackTimeout:
     def test_source_has_wait_for_timeout(self):
         """The method uses asyncio.wait_for with timeout."""
         import inspect
-        from gateway.platforms.discord import DiscordAdapter
+        from plugins.platforms.discord.adapter import DiscordAdapter
         source = inspect.getsource(DiscordAdapter.play_in_voice_channel)
         assert "wait_for" in source, \
             "play_in_voice_channel must use asyncio.wait_for for timeout"
@@ -1973,14 +1973,14 @@ class TestPlaybackTimeout:
 
     def test_playback_timeout_constant_exists(self):
         """PLAYBACK_TIMEOUT constant is defined on DiscordAdapter."""
-        from gateway.platforms.discord import DiscordAdapter
+        from plugins.platforms.discord.adapter import DiscordAdapter
         assert hasattr(DiscordAdapter, "PLAYBACK_TIMEOUT")
         assert DiscordAdapter.PLAYBACK_TIMEOUT > 0
 
     @pytest.mark.asyncio
     async def test_playback_timeout_fires(self):
         """When done event is never set, playback times out gracefully."""
-        from gateway.platforms.discord import DiscordAdapter
+        from plugins.platforms.discord.adapter import DiscordAdapter
         adapter = self._make_discord_adapter()
 
         mock_vc = MagicMock()
@@ -2008,7 +2008,7 @@ class TestPlaybackTimeout:
     @pytest.mark.asyncio
     async def test_is_playing_wait_has_timeout(self):
         """While loop waiting for previous playback has a timeout."""
-        from gateway.platforms.discord import DiscordAdapter
+        from plugins.platforms.discord.adapter import DiscordAdapter
         adapter = self._make_discord_adapter()
 
         mock_vc = MagicMock()
@@ -2124,7 +2124,7 @@ class TestVoiceChannelAwareness:
     """Tests for get_voice_channel_info() and get_voice_channel_context()."""
 
     def _make_adapter(self):
-        from gateway.platforms.discord import DiscordAdapter
+        from plugins.platforms.discord.adapter import DiscordAdapter
         from gateway.config import PlatformConfig
         config = PlatformConfig(enabled=True, extra={})
         config.token = "fake-token"
@@ -2267,7 +2267,7 @@ class TestVoiceReception:
 
     @staticmethod
     def _make_receiver(allowed_ids=None, members=None, dave=False, bot_id=9999):
-        from gateway.platforms.discord import VoiceReceiver
+        from plugins.platforms.discord.adapter import VoiceReceiver
         vc = MagicMock()
         vc._connection.secret_key = [0] * 32
         vc._connection.dave_session = MagicMock() if dave else None
@@ -2451,7 +2451,7 @@ class TestVoiceReception:
 
     def _make_receiver_with_nacl(self, dave_session=None, mapped_ssrcs=None):
         """Create a receiver that can process _on_packet with mocked NaCl + Opus."""
-        from gateway.platforms.discord import VoiceReceiver
+        from plugins.platforms.discord.adapter import VoiceReceiver
         vc = MagicMock()
         vc._connection.secret_key = [0] * 32
         vc._connection.dave_session = dave_session
@@ -2593,7 +2593,7 @@ class TestVoiceTTSPlayback:
 
     @staticmethod
     def _make_discord_adapter():
-        from gateway.platforms.discord import DiscordAdapter
+        from plugins.platforms.discord.adapter import DiscordAdapter
         from gateway.config import PlatformConfig, Platform
         config = PlatformConfig(enabled=True, extra={})
         config.token = "fake-token"
@@ -2766,14 +2766,14 @@ class TestUDPKeepalive:
     """UDP keepalive prevents Discord from dropping the voice session."""
 
     def test_keepalive_interval_is_reasonable(self):
-        from gateway.platforms.discord import DiscordAdapter
+        from plugins.platforms.discord.adapter import DiscordAdapter
         interval = DiscordAdapter._KEEPALIVE_INTERVAL
         assert 5 <= interval <= 30, f"Keepalive interval {interval}s should be between 5-30s"
 
     @pytest.mark.asyncio
     async def test_keepalive_sends_silence_frame(self):
         """Listen loop sends silence frame via send_packet after interval."""
-        from gateway.platforms.discord import DiscordAdapter
+        from plugins.platforms.discord.adapter import DiscordAdapter
         from gateway.config import PlatformConfig, Platform
 
         config = PlatformConfig(enabled=True, extra={})
@@ -2795,7 +2795,7 @@ class TestUDPKeepalive:
         adapter._voice_clients[111] = mock_vc
         mock_vc._connection = mock_conn
 
-        from gateway.platforms.discord import VoiceReceiver
+        from plugins.platforms.discord.adapter import VoiceReceiver
         mock_receiver_vc = MagicMock()
         mock_receiver_vc._connection.secret_key = [0] * 32
         mock_receiver_vc._connection.dave_session = None