feat(m2): DB foundation tests + multi-statement migration fix

- 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

.gitignore

@@ -47,6 +47,9 @@ Thumbs.db
 coverage/
 .nyc_output/
 
+# Steering files (task management - temporary)
+.steering/*
+!.steering/.gitkeep
 
 # Lock files (keep package-lock.json for consistency)
 # yarn.lock