Demote stale source_commit_missing blocker after promotion provenance is complete

[ozand]

Gap

After PR #457 deployed and one local cycle/collector pass, promotion readiness now reports source provenance complete, but /api/mission-control still headlines Blocked: source_commit_missing because the stale eeepc outbox reflection/current_blocker is still treated as authoritative.

Live evidence

• Time: 2026-05-02T14:07 MSK
• /collect: repo ok, eeepc ok, eeepc reachable=true
• eeebot-local-cycle.service: SUCCESS, evidence=evidence-2b7626e326a9
• /api/system.control_plane.promotion_replay_readiness.readiness_checks.provenance_complete=true
• /api/system.control_plane.promotion_replay_readiness.readiness_checks.missing_inputs=[]
• /api/system.control_plane.promotion_replay_readiness.readiness_reasons=[]
• /api/system.control_plane.promotion_replay_readiness.recommended_next_action=ready_for_policy_review
• Yet /api/mission-control.headline=Blocked: source_commit_missing and current_blocker.source=promotion_replay_readiness/outbox reflection-derived stale blocker.

Expected

When canonical promotion readiness has provenance complete and no missing inputs/readiness reasons, mission-control and system blocker summaries must not report source_commit_missing. They may report the next real blocker (currently #455 subagent executor misconfiguration) or promotion policy-review not-ready, but not a resolved provenance blocker.

DoD

• Regression test proving stale source_commit_missing reflection is demoted when promotion readiness inputs are complete.
• Dashboard API surfaces no source_commit_missing headline/current blocker in that state.
• Local tests pass.
• PR merged, dashboard restarted, live /api/mission-control proof collected.