feat: 添加系统命令支持 (:clear)

- 新增系统命令模块 (core/system-commands)
  - 支持 :clear/:cls/:c 清空对话历史
  - 命令注册表支持别名
  - 可扩展的命令执行器

- Server 端支持
  - 新增 /api/system-commands API
  - WebSocket 处理系统命令消息
  - 会话清空 API 端点

- UI 端支持
  - 新增 SystemCommandMenu 组件
  - 输入 : 时显示命令建议菜单
  - 键盘导航和选择
  - 底部提示添加 : 快捷键

packages/core/src/index.ts

@@ -150,6 +150,25 @@ export type {
   CommandOperationResult,
 } from './commands/index.js';
 
+// System Commands (: 前缀)
+export {
+  getSystemCommandRegistry,
+  resetSystemCommandRegistry,
+  SystemCommandExecutor,
+  isSystemCommand,
+  parseSystemCommand,
+  executeSystemCommand,
+  initializeSystemCommands,
+  builtinSystemCommands,
+} from './system-commands/index.js';
+export type {
+  SystemCommand,
+  SystemCommandContext,
+  SystemCommandResult,
+  SystemCommandAction,
+  SystemCommandInput,
+} from './system-commands/index.js';
+
 // Checkpoint
 export {
   CheckpointManager,