[codex] open guide GitHub link safely

[Sean-Kenneth-Doherty]

Summary

• make the guide page TL;DR GitHub link open like a safe external link
• add rel="noopener noreferrer" alongside target="_blank"
• keep the change scoped to the standalone guide page copy

Validation

• ran git diff --check
• ran GITHUB_TOKEN=$(gh auth token) pnpm build
• checked dist/contribute-to-opensource/index.html renders the TL;DR GitHub link with target="_blank" and rel="noopener noreferrer"

Refs #347
Refs #501