[Good first issue]: Link Careers page to GitHub good first issues

[FrankezeCode]

Summary

The Careers page describes contributor roles but does not link newcomers to open issues. Add a CTA pointing to the repo’s good first issue label filter on GitHub.

Acceptance criteria

• Add CTA text such as “Browse good first issues →” on /careers
• Links to: https://github.com/BetterDevOrg/protocol/issues?q=is%3Aissue+is%3Aopen+label%3A%22good+first+issue%22
• External link uses target="_blank" and rel="noopener noreferrer"
• Matches existing Careers page styling
• Only src/app/careers/page.tsx changed

How to test

1. Run npm run dev and open /careers
2. Confirm CTA is visible and styled consistently
3. Click link — opens GitHub issues filtered by good first issue
4. Run npm run build — passes