fix: demote resolved source commit blockers

[ozand]

Closes #458.

Summary

• Demote stale source_commit_missing current-blocker/outbox reflections once canonical promotion readiness proves provenance is complete.
• Preserve the remaining real next action (ready_for_policy_review) instead of continuing to headline the resolved provenance blocker.
• Add a dashboard/API regression that reproduces the live post-fix: derive runtime source commit from observed product head #457 inconsistency: complete promotion provenance plus stale eeepc reflection.

Test plan

• RED observed before implementation: test_api_demotes_stale_source_commit_blocker_when_promotion_provenance_is_complete failed because /api/system.control_plane.current_blocker.failure_class stayed source_commit_missing.
• PYTHONPATH=ops/dashboard/src:ops/dashboard python3 -m pytest ops/dashboard/tests/test_dashboard_truth_audit_gaps.py::test_api_demotes_stale_source_commit_blocker_when_promotion_provenance_is_complete -q → 1 passed.
• PYTHONPATH=ops/dashboard/src:ops/dashboard python3 -m pytest ops/dashboard/tests/test_dashboard_truth_audit_gaps.py -q → 68 passed.
• PYTHONPATH=ops/dashboard/src:ops/dashboard python3 -m pytest ops/dashboard/tests/test_dashboard_truth_audit_gaps.py ops/dashboard/tests/test_autonomy_stagnation_dashboard.py ops/dashboard/tests/test_app.py -q → 115 passed.
• PYTHONPATH=ops/dashboard/src:ops/dashboard python3 -m pytest ops/dashboard/tests -q → 165 passed.
• python3 -m pytest tests -q → 696 passed, 5 skipped.

Live follow-up after merge

• Restart dashboard web/collector.
• Run /collect and verify /api/mission-control.headline no longer contains source_commit_missing when readiness has provenance_complete=true and missing_inputs=[].