Align Join Us page metadata

[Hilo-Hilo]

Summary

• align the /signup/ document title with the visible Join Us navigation and llms.txt label
• add a registry regression assertion for the exact title

Pre-deployment checklist

• Reviewed this metadata change against public/llms.txt; no edit is necessary because it already says Join Us.
• public/agent-instructions.md and public/developers.md require no change.
• npm test passes: 40/40.
• npm run build passes, including SEO and machine-readable checks.
• Live verification targets identified: /signup/ and /llms.txt.