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.

## Platform Registry (gateway/platform_registry.py)
- 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

## Dynamic Platform Enum (gateway/config.py)
- 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

## Plugin Registration (hermes_cli/plugins.py)
- PluginContext.register_platform() for plugin authors
- Mirrors the existing register_tool() / register_hook() pattern

## IRC Reference Plugin (plugins/platforms/irc/)
- 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

## Tests (55 new tests)
- 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

@@ -42,6 +42,8 @@ _EXTRA_ENV_KEYS = frozenset({
     "WEIXIN_HOME_CHANNEL", "WEIXIN_HOME_CHANNEL_NAME", "WEIXIN_DM_POLICY", "WEIXIN_GROUP_POLICY",
     "WEIXIN_ALLOWED_USERS", "WEIXIN_GROUP_ALLOWED_USERS", "WEIXIN_ALLOW_ALL_USERS",
     "BLUEBUBBLES_SERVER_URL", "BLUEBUBBLES_PASSWORD",
+    "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_REPLY_MODE",
@@ -1234,6 +1236,43 @@ OPTIONAL_ENV_VARS = {
         "password": False,
         "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)",