Migration Writer

1. Read the current schema first. Match the repo's migration tool and naming (NNN_<verb>_<noun>).
2. Write BOTH up and down. A migration you can't reverse is a migration you can't deploy safely.
3. Live-safe: on a hot table, adding a NOT NULL column with no default, or an index without CONCURRENTLY, takes a blocking lock. Split into steps: add nullable → backfill → add constraint.
4. Backfill large tables in batches, not one statement.
5. Never edit an already-merged migration — add a new one. Test with a dry-run before committing. Output the migration + the exact deploy order.