Gate database migrations with a review SKILL.md

Write SKILL.md for reviewing database migrations before merge. Agents must not add destructive DDL without explicit human approval and a documented rollback.

Repository context:
[ORM or raw SQL folder e.g. prisma/migrations, drizzle, flyway, rails db/migrate]
[how to apply locally: docker compose, shadow DB, migrate command]
[production engine: Postgres, MySQL, etc.]

Migration in review (describe or paste filename):
[forward migration summary]

SKILL.md sections:
1. Purpose — schema changes ship with safety checks, not optimism.
2. When to use — any PR touching migrations/, schema.prisma, or DDL; when not to use — docs-only.
3. Required inputs: migration file names, ticket id, expected row counts or table sizes if known [human may provide], deploy window (can this lock?).
4. Review checklist the agent must complete and paste in the PR:
   - Forward migration SQL or generated steps listed in plain language.
   - Rollback: either paired down migration exists or explicit DOWN SQL in PR comment — write [missing rollback] if absent.
   - Lock risk: flag CREATE INDEX CONCURRENTLY vs blocking INDEX, ADD COLUMN DEFAULT on large tables, table rewrites.
   - Expand-contract: if removing a column or renaming, confirm two-phase plan or mark [unsafe single phase].
   - Backfill: if data backfill required, separate job or batched update with batch size and sleep — not one giant UPDATE in deploy migration unless documented.
   - Permissions: new tables have RLS/policies if the app uses them [confirm pattern].
5. Steps for the agent:
   - Read migration and related app code paths that read/write affected columns.
   - Run migrate against local or shadow DB: [paste command].
   - Run application tests touching the models: [paste command].
   - Produce PR comment block: Risk level, Rollback steps, Verify queries (e.g. \d table, SELECT count sanity).
6. Two example prompts: add nullable column vs drop column with expand-contract.
7. Pitfalls: silent type coercion, missing indexes on new FKs, running destructive ops in same deploy as code that still reads old column, assuming empty tables in prod.
8. Done means: checklist all items answered, migrate command exit 0 locally, rollback documented or escalated.

Rules:
- Do not invent table names; use only what appears in the migration paste or repo paths given.
- Do not suggest running migrations against production from the agent session.
- Markdown only.