Replace all yarn references with npm across README, CLAUDE.md,
CHANGELOG, .gitignore, and CI workflow. Remove yarn.lock.
Co-Authored-By: Claude <noreply@anthropic.com>
The CI runner does not have yarn installed. Switch to npm install
and npm run build. Also add package-lock.json for reproducible
installs and exclude it from the release ZIP.
Co-Authored-By: Claude <noreply@anthropic.com>