feat: add plugin command API for LLM-free auto-reply commands

This adds a new `api.registerCommand()` method to the plugin API, allowing
plugins to register slash commands that execute without invoking the AI agent.

Features:
- Plugin commands are processed before built-in commands and the agent
- Commands can optionally require authorization
- Commands can accept arguments
- Async handlers are supported

Use case: plugins can implement toggle commands (like /tts_on, /tts_off)
that respond immediately without consuming LLM API calls.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/plugins/runtime.ts

@@ -11,6 +11,7 @@ const createEmptyRegistry = (): PluginRegistry => ({
   httpHandlers: [],
   cliRegistrars: [],
   services: [],
+  commands: [],
   diagnostics: [],
 });