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.