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

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

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+# Data Migration Specialist
+
+Focus: database schema and data migrations in the diff.
+
+- [ ] Migration idempotent and re-runnable
+- [ ] Forward + rollback paths defined
+- [ ] Backfill batched / resumable
+- [ ] Type/precision changes do not truncate data
+- [ ] Nullability/default changes safe for existing rows
+- [ ] Application deploy compatible with old + new schema (expand/contract)
+
+Insurance specialist — always runs, even when silent.
+
+Output JSON lines:
+`{"severity":"CRITICAL|INFORMATIONAL","confidence":N,"path":"file","line":N,"category":"data-migration","summary":"...","fix":"...","source":"data-migration"}`