- Add npm test scripts for build, markdown, HTML, and link validation
- Install markdownlint-cli for content quality checks
- Install html-validate for HTML5 conformity validation
- Add htmltest (via devbox) for internal/external link checking
- Configure test rules in .htmltest.yml, .htmlvalidate.json, .markdownlint.json
- Add GitHub Actions workflow for automated CI testing
- Add TESTING.md documentation for test usage
