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/gateway/server/__tests__/test-utils.ts

@@ -12,6 +12,7 @@ export const createTestRegistry = (overrides: Partial<PluginRegistry> = {}): Plu
     httpHandlers: [],
     cliRegistrars: [],
     services: [],
+    commands: [],
     diagnostics: [],
   };
   const merged = { ...base, ...overrides };

src/gateway/test-helpers.mocks.ts

@@ -140,6 +140,7 @@ const createStubPluginRegistry = (): PluginRegistry => ({
   httpHandlers: [],
   cliRegistrars: [],
   services: [],
+  commands: [],
   diagnostics: [],
 });