test: add Vitest config and test utils

Made-with: Cursor

tests/smoke.test.ts

@@ -0,0 +1,12 @@
+import { describe, it, expect } from 'vitest';
+import { createMockDb } from './test-utils.js';
+
+describe('test-utils', () => {
+  it('createMockDb returns mock with select, insert, update, delete', () => {
+    const db = createMockDb();
+    expect(db.select).toBeDefined();
+    expect(db.insert).toBeDefined();
+    expect(db.update).toBeDefined();
+    expect(db.delete).toBeDefined();
+  });
+});