Relay Baton follow-up #2: dup chats + 405 on GET messages

Victor reported on merged #10846: (1) creating a chat adds 2 sidebar entries, only one has content; (2) Baton replies with v0 stub mock instead of using OpenClaw bridge (needs env vars in Vercel, infra not code); (3) 405 on GET /api/agent/threads/:id/messages. PR #10855 fixes (1) and (3); (2) tracked separately for env config. Branch talos/relay-baton-doublechat-and-getmessages.