Phase 0: write multi-host daemon architecture spec and repo boundary contract

[BunsDev]

Summary

Write the durable architecture spec before implementation.

Acceptance Criteria

• Includes four-device architecture diagram.
• Documents Daemon 1 hub authority over memory, loop state, job queues, and reconciliation.
• Documents stateless spoke constraints and hub-polled SSH/Tailscale communication.
• Documents MacBook Air traveling sub-daemon behavior, local-only exposure, stale cache warnings, offline queue, and reconnect handoff.
• Clearly splits implementation between OpenCoven/coven and OpenCoven/coven-cave.
• Flags conflicts with current local-socket and per-familiar SSH runtime patterns.

Notes

No code required for this phase.