docs: link GitHub and npm package

[chasechou007]

Summary

• add the alpha-aware npm package badge to both public READMEs
• lock README parity and the alpha dist-tag in the public documentation test
• pair the README change with the GitHub About Website pointing to the official npm package

Validation

• node --test tests/public-docs.test.mjs (4 passed)
• npm test (182 passed)
• npm run opendomain -- validate (26 documents, existing stale Candidate warning only)
• openspec validate --all --json (14/14)
• git diff --check