feat: 添加 :new 系统命令创建新会话

- Core: 新增 :new/:n 命令返回 new_session action
- Server: 处理 new_session action 创建新会话
- UI: useChat 添加 onSessionSwitch 回调
- Web/Desktop: ChatPage 和 App 实现会话切换逻辑

packages/web/src/App.tsx

@@ -103,6 +103,11 @@ export function App() {
     setSessionTitleUpdate({ sessionId, name });
   }, []);
 
+  // 会话切换回调（:new 命令创建新会话后切换）
+  const handleSessionSwitch = useCallback((newSessionId: string) => {
+    setCurrentSessionId(newSessionId);
+  }, []);
+
   // 处理面板宽度调整
   const handleResize = useCallback((delta: number) => {
     setIdePanelWidth((prev) => {
@@ -159,6 +164,7 @@ export function App() {
                 sessionId={currentSessionId}
                 onSessionNotFound={handleSessionNotFound}
                 onSessionUpdated={handleSessionUpdated}
+                onSessionSwitch={handleSessionSwitch}
                 responsive
                 onOpenCommands={() => setShowCommands(true)}
                 onOpenMCP={() => setShowMCP(true)}

packages/web/src/pages/Chat.tsx

@@ -22,6 +22,8 @@ interface ChatPageProps {
   sessionId: string;
   onSessionNotFound?: () => void;
   onSessionUpdated?: (sessionId: string, name: string) => void;
+  /** 切换会话回调（如 :new 命令创建新会话） */
+  onSessionSwitch?: (newSessionId: string) => void;
   responsive?: boolean;
   // 工具栏按钮
   onOpenCommands?: () => void;
@@ -39,6 +41,7 @@ export function ChatPage({
   sessionId,
   onSessionNotFound,
   onSessionUpdated,
+  onSessionSwitch,
   responsive = false,
   onOpenCommands,
   onOpenMCP,
@@ -74,6 +77,7 @@ export function ChatPage({
     },
     onSessionNotFound,
     onSessionUpdated,
+    onSessionSwitch,
     onConfigError: (error) => {
       toast.error(error.message, {
         duration: 10000,