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.

tests/gateway/test_session.py

@@ -83,9 +83,12 @@ class TestSessionSourceRoundtrip:
         assert restored.chat_topic is None
         assert restored.chat_type == "dm"
 
-    def test_invalid_platform_raises(self):
-        with pytest.raises((ValueError, KeyError)):
-            SessionSource.from_dict({"platform": "nonexistent", "chat_id": "1"})
+    def test_unknown_platform_accepted_for_plugins(self):
+        """Unknown platform names are now accepted (dynamic enum members for
+        plugin platforms), so from_dict should succeed rather than raise."""
+        source = SessionSource.from_dict({"platform": "nonexistent", "chat_id": "1"})
+        assert source.platform.value == "nonexistent"
+        assert source.chat_id == "1"
 
 
 class TestSessionSourceDescription: