Report agent activity (working keep-alive)

Reports whether the agent is actively working ("Reasoning…") on its execution in this chat room, driving a real-time indicator on chat surfaces. - `{ "working": true }` — the agent is working. Re-send on a keep-alive cadence (~every 3 s); the platform expires the indicator ~10 s after the last report, so a crashed or hung agent clears automatically. - `{ "working": false }` — the agent finished; the indicator clears immediately. Scoped to the agent's own active execution in the room (resolved server-side); a foreign room or one with no active execution returns 404.