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>

docs/plugin.md

@@ -62,6 +62,7 @@ Plugins can register:
 - Background services
 - Optional config validation
 - **Skills** (by listing `skills` directories in the plugin manifest)
+- **Auto-reply commands** (execute without invoking the AI agent)
 
 Plugins run **in‑process** with the Gateway, so treat them as trusted code.
 Tool authoring guide: [Plugin agent tools](/plugins/agent-tools).
@@ -494,6 +495,62 @@ export default function (api) {
 }
 ```
 
+### Register auto-reply commands
+
+Plugins can register custom slash commands that execute **without invoking the
+AI agent**. This is useful for toggle commands, status checks, or quick actions
+that don't need LLM processing.
+
+```ts
+export default function (api) {
+  api.registerCommand({
+    name: "mystatus",
+    description: "Show plugin status",
+    handler: (ctx) => ({
+      text: `Plugin is running! Channel: ${ctx.channel}`,
+    }),
+  });
+}
+```
+
+Command handler context:
+
+- `senderId`: The sender's ID (if available)
+- `channel`: The channel where the command was sent
+- `isAuthorizedSender`: Whether the sender is an authorized user
+- `args`: Arguments passed after the command (if `acceptsArgs: true`)
+- `commandBody`: The full command text
+- `config`: The current Clawdbot config
+
+Command options:
+
+- `name`: Command name (without the leading `/`)
+- `description`: Help text shown in command lists
+- `acceptsArgs`: Whether the command accepts arguments (default: false)
+- `requireAuth`: Whether to require authorized sender (default: false)
+- `handler`: Function that returns `{ text: string }` (can be async)
+
+Example with authorization and arguments:
+
+```ts
+api.registerCommand({
+  name: "setmode",
+  description: "Set plugin mode",
+  acceptsArgs: true,
+  requireAuth: true,
+  handler: async (ctx) => {
+    const mode = ctx.args?.trim() || "default";
+    await saveMode(mode);
+    return { text: `Mode set to: ${mode}` };
+  },
+});
+```
+
+Notes:
+- Plugin commands are processed **before** built-in commands and the AI agent
+- Commands are registered globally and work across all channels
+- Command names are case-insensitive (`/MyStatus` matches `/mystatus`)
+
 ### Register background services
 
 ```ts