fix(room): join presence channel as node listener

[itskai-dev]

Summary

• track a non-human node sentinel after the room presence channel reaches SUBSCRIBED
• keep filtering non-human presence payloads so the node listener never appears as a participant
• add a regression test covering the subscribe->track handshake

Why

The canonical narrowing now shows the cloud/browser path is live and returns 200, but the payload stays empty (participants: []). The node room subscriber was subscribing to room:${hostId} with a presence key but never calling track(). Supabase presence semantics require joining the presence set after SUBSCRIBED; otherwise the node cache can remain empty even while browsers are in the room.

Validation

• npm run build
• npx vitest run tests/room-presence-store.test.ts tests/room-presence-subscribe.test.ts tests/chat-context-room-active.test.ts