chore: restore original directory structure (project under code-review-graph-main/)

code-review-graph-main/skills/project-review/references/manual-review/data-migration.md

@@ -0,0 +1,12 @@
+# Data Migration — Manual Review Checklist
+
+High-risk module: schema/data migration requires human confirmation.
+
+- [ ] Migration is idempotent and re-runnable
+- [ ] Forward and rollback paths both defined and tested
+- [ ] Backfill is batched / resumable for large tables
+- [ ] Data type / precision changes do not silently truncate
+- [ ] Nullability and default changes safe for existing rows
+- [ ] Migration ordering across shards / replicas is consistent
+- [ ] Application deploys compatibly with both old and new schema (expand/contract)
+- [ ] Irreversible operations are flagged with a documented reason