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/routes/sessions.ts

@@ -30,7 +30,7 @@ sessionsRouter.post('/', async (c) => {
   try {
     const body = await c.req.json();
     const input = CreateSessionInputSchema.parse(body);
-    const session = sessionManager.create(input);
+    const session = await sessionManager.create(input);
 
     return c.json(
       {
@@ -76,7 +76,7 @@ sessionsRouter.get('/:id', (c) => {
 /**
  * DELETE /sessions/:id - 删除会话
  */
-sessionsRouter.delete('/:id', (c) => {
+sessionsRouter.delete('/:id', async (c) => {
   const id = c.req.param('id');
 
   if (!sessionManager.exists(id)) {
@@ -89,7 +89,7 @@ sessionsRouter.delete('/:id', (c) => {
     );
   }
 
-  sessionManager.delete(id);
+  await sessionManager.delete(id);
 
   return c.json({
     success: true,
@@ -144,7 +144,7 @@ sessionsRouter.post('/:id/messages', async (c) => {
     const body = await c.req.json();
     const input = SendMessageInputSchema.parse(body);
 
-    const message = sessionManager.addMessage(id, {
+    const message = await sessionManager.addMessage(id, {
       role: input.role,
       content: input.content,
     });