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/auto-reply/reply/commands-core.ts

@@ -24,6 +24,7 @@ import {
   handleStopCommand,
   handleUsageCommand,
 } from "./commands-session.js";
+import { handlePluginCommand } from "./commands-plugin.js";
 import type {
   CommandHandler,
   CommandHandlerResult,
@@ -31,6 +32,8 @@ import type {
 } from "./commands-types.js";
 
 const HANDLERS: CommandHandler[] = [
+  // Plugin commands are processed first, before built-in commands
+  handlePluginCommand,
   handleBashCommand,
   handleActivationCommand,
   handleSendPolicyCommand,

src/auto-reply/reply/commands-plugin.ts

@@ -0,0 +1,46 @@
+/**
+ * Plugin Command Handler
+ *
+ * Handles commands registered by plugins, bypassing the LLM agent.
+ * This handler is called before built-in command handlers.
+ */
+
+import { matchPluginCommand, executePluginCommand } from "../../plugins/commands.js";
+import type { CommandHandler, CommandHandlerResult } from "./commands-types.js";
+
+/**
+ * Handle plugin-registered commands.
+ * Returns a result if a plugin command was matched and executed,
+ * or null to continue to the next handler.
+ */
+export const handlePluginCommand: CommandHandler = async (
+  params,
+  _allowTextCommands,
+): Promise<CommandHandlerResult | null> => {
+  const { command, cfg } = params;
+
+  // Try to match a plugin command
+  const match = matchPluginCommand(command.commandBodyNormalized);
+  if (!match) return null;
+
+  // Execute the plugin command
+  const result = await executePluginCommand({
+    command: match.command,
+    args: match.args,
+    senderId: command.senderId,
+    channel: command.channel,
+    isAuthorizedSender: command.isAuthorizedSender,
+    commandBody: command.commandBodyNormalized,
+    config: cfg,
+  });
+
+  if (result) {
+    return {
+      shouldContinue: false,
+      reply: { text: result.text },
+    };
+  }
+
+  // Command was blocked (e.g., unauthorized) - don't continue to agent
+  return { shouldContinue: false };
+};