Plan and execute safe schema, data, API, and config migrations with expand/contract steps, compatibility windows, rollback, and verification. Use when changing databases, message formats, public APIs, feature cutovers, or when the user mentions migration / expand-contract / backfill / 迁移 / 兼容窗口 / 回滚迁移. API contract details defer to api-design; release cutover defers to release.
