docs: add M7-003 through M7-007 Matrix architecture sections to channel-protocol.md
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 <noreply@anthropic.com>
Jason Woltje
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