重构 Agent 聊天，支持分支管理与消息克隆

src/store/chatToolStore.ts

@@ -41,6 +41,8 @@ interface ChatToolState {
   lastAction: ChatToolAction | null;
   /** Monotonically increasing counter – lets subscribers detect new actions. */
   actionSeq: number;
+  /** Timestamp of the most recent action dispatch. */
+  lastActionAt: number;
   /** Dispatch a tool action from the chat. */
   dispatch: (action: ChatToolAction) => void;
 }
@@ -48,9 +50,11 @@ interface ChatToolState {
 export const useChatToolStore = create<ChatToolState>((set) => ({
   lastAction: null,
   actionSeq: 0,
+  lastActionAt: 0,
   dispatch: (action) =>
     set((state) => ({
       lastAction: action,
       actionSeq: state.actionSeq + 1,
+      lastActionAt: Date.now(),
     })),
 }));