refactor(storage): 统一消息存储到 Core 层

问题：Server 端只存储最终文本响应，工具调用的中间消息丢失。

解决方案：
- Agent.chat() 返回 ChatResult，包含完整消息链
- Server SessionManager 简化为只管理会话元数据
- 消息 API 改为从 Core Storage 读取
- 移除 Server 端的消息存储和 addMessage 方法

影响范围：
- core: Agent.chat() 返回类型变更
- server: SessionManager 接口变更，移除消息存储
- server: GET /sessions/:id/messages 从 Core 读取
- server: 移除 POST /sessions/:id/messages 端点

packages/server/tests/mocks/session.mock.ts

@@ -2,6 +2,8 @@
  * SessionManager Mock 工厂
  *
  * 提供会话管理器的 mock 实现
+ *
+ * 注意：消息存储已移至 Core 层，SessionManager 只负责会话元数据管理
  */
 
 import { vi } from 'vitest';
@@ -13,6 +15,7 @@ export interface MockSession {
   createdAt: number;
   updatedAt: number;
   workdir?: string;
+  messageCount: number;
 }
 
 export interface MockMessage {
@@ -27,8 +30,6 @@ export interface MockMessage {
  */
 export function createMockSessionManager() {
   const sessions = new Map<string, MockSession>();
-  const messages = new Map<string, MockMessage[]>();
-  let messageIdCounter = 1;
 
   const manager = {
     // 初始化
@@ -48,44 +49,40 @@ export function createMockSessionManager() {
         createdAt: Date.now(),
         updatedAt: Date.now(),
         workdir: data?.workdir ?? process.cwd(),
+        messageCount: 0,
       };
       sessions.set(id, session);
-      messages.set(id, []);
       return session;
     }),
 
     delete: vi.fn(async (id: string) => {
       const existed = sessions.has(id);
       sessions.delete(id);
-      messages.delete(id);
       return existed;
     }),
 
     list: vi.fn(() => Array.from(sessions.values())),
 
-    // 消息管理
-    addMessage: vi.fn(async (sessionId: string, msg: { role: 'user' | 'assistant'; content: string }) => {
-      const msgList = messages.get(sessionId) || [];
-      const newMsg: MockMessage = {
-        id: `msg-${messageIdCounter++}`,
-        role: msg.role,
-        content: msg.content,
-        timestamp: Date.now(),
-      };
-      msgList.push(newMsg);
-      messages.set(sessionId, msgList);
-      return newMsg;
-    }),
-
-    getMessages: vi.fn((id: string) => messages.get(id) || []),
-
     // 状态更新
     updateStatus: vi.fn((id: string, status: 'idle' | 'busy') => {
       const session = sessions.get(id);
       if (session) {
         session.status = status;
         session.updatedAt = Date.now();
+        return session;
       }
+      return undefined;
+    }),
+
+    // 更新消息计数
+    updateMessageCount: vi.fn((id: string, count: number) => {
+      const session = sessions.get(id);
+      if (session) {
+        session.messageCount = count;
+        session.updatedAt = Date.now();
+        return session;
+      }
+      return undefined;
     }),
 
     updateSessionName: vi.fn(async (id: string, name: string) => {
@@ -98,26 +95,20 @@ export function createMockSessionManager() {
       return null;
     }),
 
+    // Storage 访问
+    getStorage: vi.fn(() => null),
+    getProjectId: vi.fn((_sessionId: string) => 'default-project'),
+
     // 测试辅助方法
     _addSession: (session: MockSession) => {
       sessions.set(session.id, session);
-      messages.set(session.id, []);
-    },
-
-    _addMessage: (sessionId: string, message: MockMessage) => {
-      const msgList = messages.get(sessionId) || [];
-      msgList.push(message);
-      messages.set(sessionId, msgList);
     },
 
     _clear: () => {
       sessions.clear();
-      messages.clear();
-      messageIdCounter = 1;
     },
 
     _getSessions: () => sessions,
-    _getMessages: () => messages,
   };
 
   return manager;
@@ -134,6 +125,7 @@ export function createTestSession(overrides: Partial<MockSession> = {}): MockSes
     createdAt: Date.now(),
     updatedAt: Date.now(),
     workdir: '/test/workdir',
+    messageCount: 0,
     ...overrides,
   };
 }