Extends the channel protocol architecture document with five new sections: Matrix integration design (homeserver, appservice, room/space mapping, ghost users, power levels), conversation multiplexing (gateway as source of truth, real-time fanout, conflict resolution), remote auth bridging (token linking, OAuth bridge, invite provisioning, session lifecycle), agent-to-agent communication via Matrix (dedicated rooms, structured JSON payloads, coordination patterns, gateway mediation), and multi-user isolation (space-per-team, room visibility, encryption defaults, admin visibility, data retention policy). Co-Authored-By: Claude Sonnet 4.6 <[email protected]>