Migration 015 adds merge_commit_sha/squash_commit_sha to merge_requests
(Gate 4/5 prerequisites), closes_issues_synced_for_updated_at watermark
for incremental sync, and the missing idx_label_events_label index.
The MR transformer and ingestion pipeline now populate commit SHAs during
sync. The orchestrator uses watermark-based filtering for closes_issues
jobs instead of re-enqueuing all MRs every sync.
The Phase B PRD is updated to match the actual codebase: corrected
migration numbering (011-015), documented nullable label/milestone
fields (migration 012), watermark patterns (013), observability
infrastructure (014), simplified source_method values, and updated
entity_references schema to match implementation.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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