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

tests/helpers/db.ts

@@ -2,6 +2,7 @@ import os from 'node:os';
 import fs from 'node:fs';
 import path from 'node:path';
 import { SqliteDatabase } from '@/lib/db/sqlite';
+import { Migrator } from '@/lib/db/migrator';
 
 let counter = 0;
 
@@ -38,8 +39,6 @@ export function createTestDb(): SqliteDatabase {
 export function createMigratedTestDb(): SqliteDatabase {
   const db = createTestDb();
   const migrationsDir = path.join(process.cwd(), 'lib', 'db', 'migrations');
-  // eslint-disable-next-line @typescript-eslint/no-require-imports
-  const { Migrator } = require('@/lib/db/migrator');
   const migrator = new Migrator(db, migrationsDir);
   migrator.migrate();
   return db;