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.

hermes_cli/commands.py

@@ -449,7 +449,7 @@ def _iter_plugin_command_entries() -> list[tuple[str, str, str]]:
     :func:`hermes_cli.plugins.PluginContext.register_command`. They behave
     like ``CommandDef`` entries for gateway surfacing: they appear in the
     Telegram command menu, in Slack's ``/hermes`` subcommand mapping, and
-    (via :func:`gateway.platforms.discord._register_slash_commands`) in
+    (via :func:`plugins.platforms.discord.adapter._register_slash_commands`) in
     Discord's native slash command picker.
 
     Lookup is lazy so importing this module never forces plugin discovery

hermes_cli/gateway.py

@@ -3327,34 +3327,9 @@ _PLATFORMS = [
              "help": "For DMs, this is your user ID. You can set it later by typing /set-home in chat."},
         ],
     },
-    {
-        "key": "discord",
-        "label": "Discord",
-        "emoji": "💬",
-        "token_var": "DISCORD_BOT_TOKEN",
-        "setup_instructions": [
-            "1. Go to https://discord.com/developers/applications → New Application",
-            "2. Go to Bot → Reset Token → copy the bot token",
-            "3. Enable: Bot → Privileged Gateway Intents → Message Content Intent",
-            "4. Invite the bot to your server:",
-            "   OAuth2 → URL Generator → check BOTH scopes:",
-            "     - bot",
-            "     - applications.commands  (required for slash commands!)",
-            "   Bot Permissions: Send Messages, Read Message History, Attach Files",
-            "   Copy the URL and open it in your browser to invite.",
-            "5. Get your user ID: enable Developer Mode in Discord settings,",
-            "   then right-click your name → Copy ID",
-        ],
-        "vars": [
-            {"name": "DISCORD_BOT_TOKEN", "prompt": "Bot token", "password": True,
-             "help": "Paste the token from step 2 above."},
-            {"name": "DISCORD_ALLOWED_USERS", "prompt": "Allowed user IDs or usernames (comma-separated)", "password": False,
-             "is_allowlist": True,
-             "help": "Paste your user ID from step 5 above."},
-            {"name": "DISCORD_HOME_CHANNEL", "prompt": "Home channel ID (for cron/notification delivery, or empty to set later with /set-home)", "password": False,
-             "help": "Right-click a channel → Copy Channel ID (requires Developer Mode)."},
-        ],
-    },
+    # Discord moved to plugins/platforms/discord/ — its setup metadata is
+    # discovered dynamically via _all_platforms() from the platform registry
+    # entry registered by plugins/platforms/discord/adapter.py::register().
     {
         "key": "slack",
         "label": "Slack",
@@ -4747,7 +4722,9 @@ def _builtin_setup_fn(key: str):
     from hermes_cli import setup as _s
     return {
         "telegram": _s._setup_telegram,
-        "discord": _s._setup_discord,
+        # discord moved into the plugin: setup_fn is registered by
+        # plugins/platforms/discord/adapter.py::register() and dispatched
+        # via the plugin path in _configure_platform().
         "slack": _s._setup_slack,
         "matrix": _s._setup_matrix,
         "mattermost": _s._setup_mattermost,

hermes_cli/setup.py

@@ -2034,74 +2034,6 @@ def _setup_telegram():
             save_env_value("TELEGRAM_HOME_CHANNEL", home_channel)
 
 
-def _setup_discord():
-    """Configure Discord bot credentials and allowlist."""
-    print_header("Discord")
-    existing = get_env_value("DISCORD_BOT_TOKEN")
-    if existing:
-        print_info("Discord: already configured")
-        if not prompt_yes_no("Reconfigure Discord?", False):
-            if not get_env_value("DISCORD_ALLOWED_USERS"):
-                print_info("⚠️  Discord has no user allowlist - anyone can use your bot!")
-                if prompt_yes_no("Add allowed users now?", True):
-                    print_info("   To find Discord ID: Enable Developer Mode, right-click name → Copy ID")
-                    allowed_users = prompt("Allowed user IDs (comma-separated)")
-                    if allowed_users:
-                        cleaned_ids = _clean_discord_user_ids(allowed_users)
-                        save_env_value("DISCORD_ALLOWED_USERS", ",".join(cleaned_ids))
-                        print_success("Discord allowlist configured")
-            return
-
-    print_info("Create a bot at https://discord.com/developers/applications")
-    token = prompt("Discord bot token", password=True)
-    if not token:
-        return
-    save_env_value("DISCORD_BOT_TOKEN", token)
-    print_success("Discord token saved")
-
-    print()
-    print_info("🔒 Security: Restrict who can use your bot")
-    print_info("   To find your Discord user ID:")
-    print_info("   1. Enable Developer Mode in Discord settings")
-    print_info("   2. Right-click your name → Copy ID")
-    print()
-    print_info("   You can also use Discord usernames (resolved on gateway start).")
-    print()
-    allowed_users = prompt(
-        "Allowed user IDs or usernames (comma-separated, leave empty for open access)"
-    )
-    if allowed_users:
-        cleaned_ids = _clean_discord_user_ids(allowed_users)
-        save_env_value("DISCORD_ALLOWED_USERS", ",".join(cleaned_ids))
-        print_success("Discord allowlist configured")
-    else:
-        print_info("⚠️  No allowlist set - anyone in servers with your bot can use it!")
-
-    print()
-    print_info("📬 Home Channel: where Hermes delivers cron job results,")
-    print_info("   cross-platform messages, and notifications.")
-    print_info("   To get a channel ID: right-click a channel → Copy Channel ID")
-    print_info("   (requires Developer Mode in Discord settings)")
-    print_info("   You can also set this later by typing /set-home in a Discord channel.")
-    home_channel = prompt("Home channel ID (leave empty to set later with /set-home)")
-    if home_channel:
-        save_env_value("DISCORD_HOME_CHANNEL", home_channel)
-
-
-def _clean_discord_user_ids(raw: str) -> list:
-    """Strip common Discord mention prefixes from a comma-separated ID string."""
-    cleaned = []
-    for uid in raw.replace(" ", "").split(","):
-        uid = uid.strip()
-        if uid.startswith("<@") and uid.endswith(">"):
-            uid = uid.lstrip("<@!").rstrip(">")
-        if uid.lower().startswith("user:"):
-            uid = uid[5:]
-        if uid:
-            cleaned.append(uid)
-    return cleaned
-
-
 def _setup_slack():
     """Configure Slack bot credentials."""
     print_header("Slack")