feat: pluggable platform adapter registry + IRC reference implementation

Adds a platform adapter plugin interface so anyone can create new gateway
platforms (IRC, Viber, Line, etc.) as drop-in plugins without modifying
core gateway code.

- PlatformEntry dataclass: name, label, adapter_factory, check_fn,
  validate_config, required_env, install_hint, source
- PlatformRegistry singleton with register/unregister/create_adapter
- _create_adapter() in gateway/run.py checks registry first, falls
  through to existing if/elif chain for built-in platforms

- Platform._missing_() accepts unknown string values, creating cached
  pseudo-members so Platform('irc') is Platform('irc') holds true
- GatewayConfig.from_dict() now parses plugin platform names from
  config.yaml without rejecting them
- get_connected_platforms() delegates to registry for unknown platforms

- PluginContext.register_platform() for plugin authors
- Mirrors the existing register_tool() / register_hook() pattern

- Full async IRC adapter using stdlib asyncio (zero external deps)
- Connects via TLS, handles PING/PONG, nick collision, NickServ auth
- Channel messages require addressing (nick: msg), DMs always dispatch
- Markdown stripping for IRC-clean output, message splitting for
  512-byte line limit
- Config via config.yaml extra dict or IRC_* env vars

- Platform enum dynamic members (identity stability, case normalization)
- PlatformRegistry (register, unregister, create, validation, factory)
- GatewayConfig integration (from_dict parsing, get_connected_platforms)
- IRC adapter (init, send, protocol parsing, markdown, requirements)

No existing platform adapters were migrated — the if/elif chain is
untouched. This is Phase 1: prove the interface with a real plugin.

hermes_cli/config.py

@@ -73,6 +73,8 @@ _EXTRA_ENV_KEYS = frozenset({
     "QQ_HOME_CHANNEL", "QQ_HOME_CHANNEL_NAME",  # legacy aliases (pre-rename, still read for back-compat)
     "QQ_ALLOWED_USERS", "QQ_GROUP_ALLOWED_USERS", "QQ_ALLOW_ALL_USERS", "QQ_MARKDOWN_SUPPORT",
     "QQ_STT_API_KEY", "QQ_STT_BASE_URL", "QQ_STT_MODEL",
+    "IRC_SERVER", "IRC_PORT", "IRC_NICKNAME", "IRC_CHANNEL",
+    "IRC_USE_TLS", "IRC_SERVER_PASSWORD", "IRC_NICKSERV_PASSWORD",
     "TERMINAL_ENV", "TERMINAL_SSH_KEY", "TERMINAL_SSH_PORT",
     "WHATSAPP_MODE", "WHATSAPP_ENABLED",
     "MATTERMOST_HOME_CHANNEL", "MATTERMOST_HOME_CHANNEL_NAME", "MATTERMOST_REPLY_MODE",
@@ -2084,6 +2086,43 @@ OPTIONAL_ENV_VARS = {
         "prompt": "QQ Sandbox Mode",
         "category": "messaging",
     },
+    "IRC_SERVER": {
+        "description": "IRC server hostname (e.g. irc.libera.chat)",
+        "prompt": "IRC server",
+        "url": None,
+        "password": False,
+        "category": "messaging",
+    },
+    "IRC_CHANNEL": {
+        "description": "IRC channel to join (e.g. #hermes)",
+        "prompt": "IRC channel",
+        "url": None,
+        "password": False,
+        "category": "messaging",
+    },
+    "IRC_NICKNAME": {
+        "description": "Bot nickname on IRC (default: hermes-bot)",
+        "prompt": "IRC nickname",
+        "url": None,
+        "password": False,
+        "category": "messaging",
+    },
+    "IRC_SERVER_PASSWORD": {
+        "description": "IRC server password (if required)",
+        "prompt": "IRC server password",
+        "url": None,
+        "password": True,
+        "category": "messaging",
+        "advanced": True,
+    },
+    "IRC_NICKSERV_PASSWORD": {
+        "description": "NickServ password for nick identification",
+        "prompt": "NickServ password",
+        "url": None,
+        "password": True,
+        "category": "messaging",
+        "advanced": True,
+    },
     "GATEWAY_ALLOW_ALL_USERS": {
         "description": "Allow all users to interact with messaging bots (true/false). Default: false.",
         "prompt": "Allow all users (true/false)",

hermes_cli/plugins.py

@@ -445,6 +445,53 @@ class PluginContext:
             self.manifest.name, provider.name,
         )
 
+    # -- platform adapter registration ---------------------------------------
+
+    def register_platform(
+        self,
+        name: str,
+        label: str,
+        adapter_factory: Callable,
+        check_fn: Callable,
+        validate_config: Callable | None = None,
+        required_env: list | None = None,
+        install_hint: str = "",
+    ) -> None:
+        """Register a gateway platform adapter.
+
+        The adapter_factory receives a ``PlatformConfig`` and returns a
+        ``BasePlatformAdapter`` subclass instance.  The gateway calls
+        ``check_fn()`` before instantiation to verify dependencies.
+
+        Example::
+
+            ctx.register_platform(
+                name="irc",
+                label="IRC",
+                adapter_factory=lambda cfg: IRCAdapter(cfg),
+                check_fn=lambda: True,
+            )
+        """
+        from gateway.platform_registry import platform_registry, PlatformEntry
+
+        entry = PlatformEntry(
+            name=name,
+            label=label,
+            adapter_factory=adapter_factory,
+            check_fn=check_fn,
+            validate_config=validate_config,
+            required_env=required_env or [],
+            install_hint=install_hint,
+            source="plugin",
+        )
+        platform_registry.register(entry)
+        self._manager._plugin_platform_names.add(name)
+        logger.debug(
+            "Plugin %s registered platform: %s",
+            self.manifest.name,
+            name,
+        )
+
     # -- hook registration --------------------------------------------------
 
     def register_hook(self, hook_name: str, callback: Callable) -> None:
@@ -523,6 +570,7 @@ class PluginManager:
         self._plugins: Dict[str, LoadedPlugin] = {}
         self._hooks: Dict[str, List[Callable]] = {}
         self._plugin_tool_names: Set[str] = set()
+        self._plugin_platform_names: Set[str] = set()
         self._cli_commands: Dict[str, dict] = {}
         self._context_engine = None  # Set by a plugin via register_context_engine()
         self._plugin_commands: Dict[str, dict] = {}  # Slash commands registered by plugins