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.

plugins/platforms/irc/PLUGIN.yaml

@@ -0,0 +1,12 @@
+name: irc-platform
+version: 1.0.0
+description: >
+  IRC gateway adapter for Hermes Agent.
+  Connects to an IRC server and relays messages between an IRC channel
+  (or DMs) and the Hermes agent.  No external dependencies — uses
+  Python's stdlib asyncio for the IRC protocol.
+author: Nous Research
+requires_env:
+  - IRC_SERVER
+  - IRC_CHANNEL
+  - IRC_NICKNAME