feat(server): 添加 session 持久化支持

- 复用 core 包的 SessionStorage 实现文件持久化
- sessions 保存到 ~/.local/share/ai-assist/sessions/
- 服务启动时自动加载已持久化的 sessions
- create/addMessage/delete 操作自动同步到文件

packages/server/src/agent/adapter.ts

@@ -148,7 +148,7 @@ export async function processMessage(sessionId: string, content: string): Promis
       },
     });
 
-    const assistantMessage = sessionManager.addMessage(sessionId, {
+    const assistantMessage = await sessionManager.addMessage(sessionId, {
       role: 'assistant',
       content: 'Agent core module not available. Please build @ai-assistant/core first.',
     });
@@ -184,7 +184,7 @@ export async function processMessage(sessionId: string, content: string): Promis
     });
 
     // 保存助手消息
-    const assistantMessage = sessionManager.addMessage(sessionId, {
+    const assistantMessage = await sessionManager.addMessage(sessionId, {
       role: 'assistant',
       content: response,
     });