- Add SqliteDatabase.exec() using better-sqlite3 exec() so the migrator
can run multi-statement SQL files (prepare() rejected 001_initial.sql)
- Migrator now uses exec() for migration file content
- Fix tests/helpers/db.ts: replace CJS require() with ESM import
- Add unit tests for SqliteDatabase (query/get/execute/transaction/exec,
WAL & foreign_keys pragmas) and Migrator (ordering, idempotency,
rollback, real 001_initial.sql applies)
- Restore .gitignore steering exclusion; remove stale steering files