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/src/agent/adapter.ts

@@ -39,12 +39,20 @@ export interface CompressionResult {
   summaryTokens?: number;
 }
 
+/**
+ * Chat 返回结果
+ */
+interface ChatResult {
+  text: string;
+  messages: unknown[];
+}
+
 /**
  * Agent 实例接口
  */
 interface AgentInstance {
   setRegistry(registry: unknown): void;
-  chat(message: string, onStream?: (chunk: string) => void): Promise<string>;
+  chat(message: string, onStream?: (chunk: string) => void): Promise<ChatResult>;
   getToolCount(): { core: number; discovered: number; total: number };
   getContextUsageFormatted(): string;
   getContextUsage(): TokenUsage;
@@ -52,6 +60,7 @@ interface AgentInstance {
   getCompressionManager(): {
     shouldCompress(messages: unknown[]): boolean;
   };
+  getHistory(): unknown[];
 }
 
 /**
@@ -256,23 +265,17 @@ export async function processMessage(sessionId: string, content: string): Promis
     }
 
     // Core 模块不可用，返回占位响应
+    const errorContent = 'Agent core module not available. Please build @ai-assistant/core first.';
     broadcastToSession(sessionId, {
       type: 'chunk',
       sessionId,
-      payload: {
-        content: 'Agent core module not available. Please build @ai-assistant/core first.',
-      },
-    });
-
-    const assistantMessage = await sessionManager.addMessage(sessionId, {
-      role: 'assistant',
-      content: 'Agent core module not available. Please build @ai-assistant/core first.',
+      payload: { content: errorContent },
     });
 
     broadcastToSession(sessionId, {
       type: 'done',
       sessionId,
-      payload: assistantMessage,
+      payload: { text: errorContent, hasToolCalls: false, messageCount: 0 },
     });
 
     sessionManager.updateStatus(sessionId, 'idle' as SessionStatus);
@@ -282,7 +285,7 @@ export async function processMessage(sessionId: string, content: string): Promis
 
   try {
     // 调用 Agent 的 chat 方法，使用流式回调
-    const response = await agent.chat(content, (chunk: string) => {
+    const result = await agent.chat(content, (chunk: string) => {
       // 推送流式内容
       broadcastToSession(sessionId, {
         type: 'chunk',
@@ -299,31 +302,41 @@ export async function processMessage(sessionId: string, content: string): Promis
       }
     });
 
-    // 保存助手消息
-    const assistantMessage = await sessionManager.addMessage(sessionId, {
-      role: 'assistant',
-      content: response,
+    // 消息已由 Core Agent 自动持久化，这里只更新 Server 端的会话计数
+    const session = sessionManager.get(sessionId);
+    if (session) {
+      // 从 Agent 获取实际消息数
+      const history = agent.getHistory();
+      session.messageCount = history.length;
+      session.updatedAt = new Date().toISOString();
+    }
+
+    // 检查是否有工具调用
+    const hasToolCalls = result.messages.some((m: unknown) => {
+      const msg = m as { content?: unknown };
+      return Array.isArray(msg.content) && msg.content.some((c: unknown) => {
+        const block = c as { type?: string };
+        return block.type === 'tool-call';
+      });
     });
 
     // 发送完成消息
     broadcastToSession(sessionId, {
       type: 'done',
       sessionId,
-      payload: assistantMessage,
+      payload: {
+        text: result.text,
+        hasToolCalls,
+        messageCount: result.messages.length,
+      },
     });
 
     // 检查是否需要生成会话标题（首次对话完成后）
-    const session = sessionManager.get(sessionId);
-    const messages = sessionManager.getMessages(sessionId);
-    if (session && !session.name && messages.length === 2) {
-      // 首条用户消息 + 首条 AI 回复 = 2 条消息
-      const userMessage = messages.find(m => m.role === 'user');
-      if (userMessage) {
-        // 异步生成标题，不阻塞响应
-        generateSessionTitle(sessionId, userMessage.content, response).catch(err => {
-          console.error('[Agent] Failed to generate session title:', err);
-        });
-      }
+    if (session && !session.name) {
+      // 异步生成标题，不阻塞响应
+      generateSessionTitle(sessionId, content, result.text).catch(err => {
+        console.error('[Agent] Failed to generate session title:', err);
+      });
     }
 
     emitStatusEvent(sessionId, 'idle');