fix: 过滤 subagent session 避免污染会话列表

subagent session（id 形如 sub:...）存入 DB 时 channel_name=websocket，会被 list_sessions 查出并显示在前端右上角。SQL 添加 AND id NOT LIKE 'sub:%' 过滤。

src/storage/mod.rs

@@ -376,6 +376,7 @@ impl SessionStore {
             FROM sessions
             WHERE channel_name = ?1
               AND deleted_at IS NULL
+              AND id NOT LIKE 'sub:%'
             ",
         );