[codex] use relative guide link

[Sean-Kenneth-Doherty]

Summary

• change the homepage hero guide link from the production URL to the app-relative guide path
• keep local builds and PR previews on the same origin when users click the guide link

Refs #501

Validation

• GITHUB_TOKEN=$(gh auth token) pnpm build
• checked generated dist/index.html has one href="/contribute-to-opensource" hero link and no production-domain guide href
• checked generated dist/contribute-to-opensource/index.html exists and contains the guide page
• git diff --check main...HEAD

[Sean-Kenneth-Doherty]

Validated this head in a detached scratch worktree (f65fb5ee105d652c353b78ac4f9b39b99c011f53).

Commands/checks run:

GITHUB_TOKEN=$(gh auth token) pnpm build
rg -n 'href="/contribute-to-opensource"' dist/index.html
rg -n 'https://firstcontributions.github.io/contribute-to-opensource' dist/index.html  # expected no matches
test -f dist/contribute-to-opensource/index.html
git diff --check

Result: pnpm build completed successfully. The generated homepage has the app-relative /contribute-to-opensource hero link, no production-domain guide link, and the generated guide page exists. git diff --check was clean.