XY-611: Handle Linear rate limits and retained post-review active-label drift without stranding green lanes

[yvette-carlisle]

Summary

• persist project-scoped Linear connector backoff and honor it in run/status/serve/recovery paths
• preserve retained review handoff rows from local PR markers when Linear readback is degraded
• classify missing active ownership on bound review handoff diagnostics as repairable ownership drift

Verification

• cargo make fmt
• cargo make lint-fix
• cargo make test

Review

• implementation pass: requirements covered across backoff, retained-row visibility, status separation, recovery diagnosis, and docs
• adversarial pass: repaired control-plane backoff clearing so degraded snapshots do not immediately clear persisted Linear backoff