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

lib/db/migrator.ts

@@ -37,7 +37,7 @@ export class Migrator {
       const sql = fs.readFileSync(fullPath, 'utf-8');
 
       this.db.transaction(() => {
-        this.db.execute(sql);
+        this.db.exec(sql);
         this.db.execute(
           `INSERT INTO schema_migrations (filename, applied_at) VALUES (@filename, @appliedAt)`,
           { filename: file, appliedAt: new Date().toISOString() }