fix: include Nitrogen generated files in package

[jingjing2222]

Summary

• Commit nitrogen/generated outputs so installs from GitHub include the Nitro bridge without consumer-side generation.
• Stop ignoring nitrogen/ and update contributor docs to match Nitro's packaging guidance.
• Add verify:package-contents to assert the npm tarball includes built JS and key Nitrogen generated files.
• Add CI checks for generated Nitrogen drift and package contents.

Root Cause

The npm tarball could include generated files because prepack runs yarn build, but the Git repository did not track nitrogen/. Direct GitHub installs therefore received native build files that reference generated Nitrogen paths without those files being present.