feat(server): 添加统一命令系统 API

- 新增 /api/commands 路由，支持列出、查询、执行和搜索命令
- Server 通过动态导入 Core 的 CommandRegistry 和 CommandExecutor
- CLI、Web、Desktop 客户端均可通过 REST API 访问斜杠命令
- 支持三层命令优先级: project > user > builtin

packages/server/src/routes/index.ts

@@ -8,3 +8,4 @@ export { sessionsRouter } from './sessions.js';
 export { toolsRouter, registerTool, getRegisteredTools } from './tools.js';
 export { configRouter, getConfig, setConfig } from './config.js';
 export { filesRouter, setWorkingDirectory, getWorkingDirectory } from './files.js';
+export { commandsRouter } from './commands.js';