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/tools/test_send_message_tool.py

@@ -28,16 +28,93 @@ def _reset_signal_scheduler():
 
 from gateway.config import Platform
 from tools.send_message_tool import (
-    _derive_forum_thread_name,
     _is_telegram_thread_not_found,
     _parse_target_ref,
-    _send_discord,
     _send_matrix_via_adapter,
     _send_signal,
     _send_telegram,
     _send_to_platform,
     send_message_tool,
 )
+# Discord helpers moved to the plugin in #24325.  Import from the new path
+# and provide a thin ``_send_discord(token, ...)`` shim that mirrors the
+# pre-migration signature so the existing test bodies keep working.
+from plugins.platforms.discord.adapter import (
+    _DISCORD_CHANNEL_TYPE_PROBE_CACHE,
+    _derive_forum_thread_name,
+    _probe_is_forum_cached,
+    _remember_channel_is_forum,
+    _standalone_send,
+)
+
+
+async def _send_discord(
+    token,
+    chat_id,
+    message,
+    *,
+    thread_id=None,
+    media_files=None,
+):
+    """Pre-migration ``(token, chat_id, message, …)`` adapter around the
+    plugin's ``_standalone_send(pconfig, …)``.  Lets test bodies continue
+    to call ``_send_discord("tok", ...)`` without rewriting every signature.
+    """
+    pconfig = SimpleNamespace(token=token, extra={})
+    return await _standalone_send(
+        pconfig,
+        chat_id,
+        message,
+        thread_id=thread_id,
+        media_files=media_files,
+    )
+
+
+def _discord_entry():
+    """Return the live Discord PlatformEntry, importing lazily so plugin
+    discovery is forced exactly once and patches survive across tests."""
+    from hermes_cli.plugins import discover_plugins
+    from gateway.platform_registry import platform_registry
+    discover_plugins()
+    return platform_registry.get("discord")
+
+
+class _patch_discord_sender:
+    """Patch the Discord registry entry's ``standalone_sender_fn`` with the
+    given mock and translate the production ``(pconfig, ...)`` call shape
+    back to the pre-migration ``(token, ...)`` shape the test mocks expect.
+
+    Use as a context manager:
+
+        send_mock = AsyncMock(return_value={...})
+        with _patch_discord_sender(send_mock):
+            asyncio.run(_send_to_platform(Platform.DISCORD, ...))
+        send_mock.assert_awaited_once_with("tok", "chat", "msg",
+                                           thread_id=None, media_files=[])
+    """
+
+    def __init__(self, mock):
+        self._mock = mock
+        self._entry = None
+        self._original = None
+
+    async def _adapter(self, pconfig, chat_id, message, *, thread_id=None, media_files=None):
+        token = getattr(pconfig, "token", None)
+        return await self._mock(
+            token, chat_id, message,
+            thread_id=thread_id, media_files=media_files,
+        )
+
+    def __enter__(self):
+        self._entry = _discord_entry()
+        self._original = self._entry.standalone_sender_fn
+        self._entry.standalone_sender_fn = self._adapter
+        return self._mock
+
+    def __exit__(self, exc_type, exc, tb):
+        if self._entry is not None:
+            self._entry.standalone_sender_fn = self._original
+        return False
 
 
 def _run_async_immediately(coro):
@@ -446,7 +523,7 @@ class TestSendToPlatformChunking:
         """Messages exceeding the platform limit are split into multiple sends."""
         send = AsyncMock(return_value={"success": True, "message_id": "1"})
         long_msg = "word " * 1000  # ~5000 chars, well over Discord's 2000 limit
-        with patch("tools.send_message_tool._send_discord", send):
+        with _patch_discord_sender(send):
             result = asyncio.run(
                 _send_to_platform(
                     Platform.DISCORD,
@@ -1176,7 +1253,7 @@ class TestSendToPlatformDiscordThread:
         """Discord platform with thread_id passes it to _send_discord."""
         send_mock = AsyncMock(return_value={"success": True, "message_id": "1"})
 
-        with patch("tools.send_message_tool._send_discord", send_mock):
+        with _patch_discord_sender(send_mock):
             result = asyncio.run(
                 _send_to_platform(
                     Platform.DISCORD,
@@ -1196,7 +1273,7 @@ class TestSendToPlatformDiscordThread:
         """Discord platform without thread_id passes None."""
         send_mock = AsyncMock(return_value={"success": True, "message_id": "1"})
 
-        with patch("tools.send_message_tool._send_discord", send_mock):
+        with _patch_discord_sender(send_mock):
             result = asyncio.run(
                 _send_to_platform(
                     Platform.DISCORD,
@@ -1360,7 +1437,7 @@ class TestSendToPlatformDiscordMedia:
         # A message long enough to get chunked (Discord limit is 2000)
         long_msg = "A" * 1900 + " " + "B" * 1900
 
-        with patch("tools.send_message_tool._send_discord", side_effect=mock_send_discord):
+        with _patch_discord_sender(AsyncMock(side_effect=mock_send_discord)):
             result = asyncio.run(
                 _send_to_platform(
                     Platform.DISCORD,
@@ -1380,7 +1457,7 @@ class TestSendToPlatformDiscordMedia:
         """Short message (single chunk) gets media_files directly."""
         send_mock = AsyncMock(return_value={"success": True, "message_id": "1"})
 
-        with patch("tools.send_message_tool._send_discord", send_mock):
+        with _patch_discord_sender(send_mock):
             result = asyncio.run(
                 _send_to_platform(
                     Platform.DISCORD,
@@ -1618,7 +1695,7 @@ class TestSendToPlatformDiscordForum:
         """Discord messages are routed through _send_discord, which handles forum detection."""
         send_mock = AsyncMock(return_value={"success": True, "message_id": "1"})
 
-        with patch("tools.send_message_tool._send_discord", send_mock):
+        with _patch_discord_sender(send_mock):
             result = asyncio.run(
                 _send_to_platform(
                     Platform.DISCORD,
@@ -1637,7 +1714,7 @@ class TestSendToPlatformDiscordForum:
         """Thread ID is still passed through when sending to Discord."""
         send_mock = AsyncMock(return_value={"success": True, "message_id": "1"})
 
-        with patch("tools.send_message_tool._send_discord", send_mock):
+        with _patch_discord_sender(send_mock):
             result = asyncio.run(
                 _send_to_platform(
                     Platform.DISCORD,
@@ -1775,11 +1852,11 @@ class TestForumProbeCache:
     """_DISCORD_CHANNEL_TYPE_PROBE_CACHE memoizes forum detection results."""
 
     def setup_method(self):
-        from tools import send_message_tool as smt
-        smt._DISCORD_CHANNEL_TYPE_PROBE_CACHE.clear()
+        from plugins.platforms.discord import adapter as discord_adapter
+        discord_adapter._DISCORD_CHANNEL_TYPE_PROBE_CACHE.clear()
 
     def test_cache_round_trip(self):
-        from tools.send_message_tool import (
+        from plugins.platforms.discord.adapter import (
             _probe_is_forum_cached,
             _remember_channel_is_forum,
         )
@@ -1819,7 +1896,7 @@ class TestForumProbeCache:
         thread_session.post = MagicMock(return_value=thread_resp)
 
         # Two _send_discord calls: first does probe + thread-create; second should skip probe
-        from tools import send_message_tool as smt
+        from plugins.platforms.discord import adapter as discord_adapter
 
         sessions_created = []
 
@@ -1837,7 +1914,7 @@ class TestForumProbeCache:
         with patch("aiohttp.ClientSession", side_effect=session_factory):
             result1 = asyncio.run(_send_discord("tok", "ch1", "first"))
         assert result1["success"] is True
-        assert smt._probe_is_forum_cached("ch1") is True
+        assert discord_adapter._probe_is_forum_cached("ch1") is True
 
         # Second call: cache hits, no new probe session needed. We need to only
         # return thread_session now since probe is skipped.