CI/CD - Add 'main', and 'dev' branch github action hooks for pull requests #118

.github/workflows/monorepo-build.yml still triggers only on the monorepo
branch (push.branches: [ monorepo ] and pull_request.branches: [ monorepo ]),
but #76 promoted monorepo to main — so PRs into main/dev currently run
no build at all.

Acceptance criteria

• Update both triggers to [ main, dev ]
• Confirm the matrix (18.x / 20.x / 22.X) is still the intended support
  range
• Make the build a required status check on main