Monorepo and ESM (#58)

[mwarman]

🔧 Change Summary

Closes #58

• Converted the project to an npm workspaces monorepo:
  • packages/api holds the Lambda handlers and all application source.
  • packages/infra holds the AWS CDK infrastructure.
  • Shared dependencies and the common scripts (build, clean, test, test:coverage) live in the root package.json and run in each workspace with --workspaces --if-present. There is a single root package-lock.json.
  • Added a root tsconfig.base.json that both workspaces extend.
• Switched the whole project to ESM and upgraded TypeScript to 6.0.3:
  • All packages use "type": "module". Relative and alias imports now use .js extensions.
  • The @/ alias is kept per workspace (packages/api/src for the api, the package root for infra). It is configured in each tsconfig, in each Vitest config, and for esbuild via the API tsconfig.
  • The Lambdas are bundled as ESM (index.mjs) with a createRequire banner. lambda-stack.ts no longer uses __dirname.
  • The CDK app runs with tsx instead of ts-node. source-map-support was removed.
  • The api build script is now tsc --noEmit, because the Lambdas are bundled by esbuild during CDK synth and deploy.
• Migrated all tests from Jest to Vitest:
  • There is a base vitest.config.ts at the root, extended in each workspace with mergeConfig.
  • Mocks now use vi.hoisted, and require() calls became await import().
  • Two test setups changed beyond the mechanical migration. task-service.test.ts now uses fake timers instead of a Date spy, and sns-client.test.ts mocks @leanstacks/lambda-utils. Test names and assertions are unchanged.
• Updated the GitHub Actions workflows to use a single root npm ci, with the CDK steps running in packages/infra.
• Updated the documentation (README, docs/, packages/infra/README.md) for the new structure, scripts, ESM conventions, and Vitest.
• Added AGENTS.md. .github/copilot-instructions.md now points to it.

📝 Checklist

• Complete a self-review of changes
• Unit tests have been created or updated
• The code is free of [new] lint errors and warnings
• Update project documentation as needed: README, /docs, JSDoc, etc.

🧪 Steps to Test

1. Run npm ci from the repository root.
2. Run npm run lint, npm run format:check, npm run build, and npm run test:coverage. All should pass.
3. Create packages/infra/.env from .env.example, then run npm run synth -w packages/infra. Verify that each Lambda bundles as index.mjs under packages/infra/cdk.out.
4. Optionally deploy to LocalStack (npm run local:start, then the local:* scripts in packages/infra) and exercise the API.

🔗 Additional Information

• Not yet verified: the GitHub Actions workflows and a LocalStack deployment. Local cdk synth and loading a bundled handler in Node both worked.
• deploy-reusable.yml runs npm run bootstrap, which is not a script in the infra package. That predates this change and I left it alone.