fix(gui): 修复启动报错和对话列表不正确的两个关键Bug

Bug1: 'async for' requires __aiter__ method, got coroutine
- EventQueue.subscribe() 在 _closed=True 时直接 return，
  Python 将其视为协程而非异步生成器
- 修复: 添加不可达的 yield 语句，确保函数始终为异步生成器

Bug2: 启动时对话列表全显示"对话"，无法识别之前的对话
- list_conversations() 不加载消息，_derive_conversation_title
  遍历空 messages 列表导致标题全为"对话"
- 修复: list_conversations 从 SQLite 加载首条用户消息用于标题推导

Bug2b: WebSocket 不响应前端对话切换
- conv 变量只在首条消息时设置，之后忽略 conversation_id
- 修复: 每条消息都检查 conversation_id，切换时更新 conv

src/agentkit/chat/sqlite_conversation_store.py

@@ -276,13 +276,25 @@ class SqliteConversationStore:
             if conv_id in self._cache:
                 result.append(self._cache[conv_id])
             else:
-                result.append(
-                    Conversation(
+                conv = Conversation(
                     id=conv_id,
                     created_at=self._str_to_dt(row["created_at"]),
                     updated_at=self._str_to_dt(row["updated_at"]),
                 )
+                # Load first user message from SQLite for title derivation
+                try:
+                    title_cursor = await db.execute(
+                        "SELECT content FROM messages "
+                        "WHERE conversation_id = ? AND role = 'user' "
+                        "ORDER BY timestamp ASC LIMIT 1",
+                        (conv_id,),
                     )
+                    title_row = await title_cursor.fetchone()
+                    if title_row:
+                        conv.messages.append(ChatMessage(role="user", content=title_row["content"]))
+                except Exception:
+                    pass
+                result.append(conv)
         return result
 
     async def restore_from_store(

src/agentkit/core/event_queue.py

@@ -210,7 +210,9 @@ class EventQueue:
         保证不会遗漏或重复事件。
         """
         if self._closed:
+            # Must yield to make this an async generator, not a coroutine
             return
+            yield  # noqa: unreachable — makes this an async generator
 
         # P1 #13 fix: enforce subscriber cap to prevent resource exhaustion
         # from malicious resume floods or runaway client loops.

src/agentkit/server/routes/portal.py

@@ -1141,9 +1141,13 @@ async def portal_websocket(websocket: WebSocket):
             if not message_text:
                 continue
 
-            # Create conversation on first message (not on connect)
-            if conv is None:
+            # Create or switch conversation based on conversation_id from frontend
             conv_id = msg.get("conversation_id")
+            if conv_id:
+                if conv is None or conv.id != conv_id:
+                    conv = await _conversation_store.get_or_create(conv_id)
+                    await websocket.send_json({"type": "connected", "conversation_id": conv.id})
+            elif conv is None:
                 conv = await _conversation_store.get_or_create(conv_id)
                 await websocket.send_json({"type": "connected", "conversation_id": conv.id})