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>
