Fix deploy reruns by retaining github-pages artifact in render workflow

[Copilot]

The deploy GitHub Actions job was failing on reruns because actions/deploy-pages could not find the github-pages artifact from the earlier render job attempt. This PR updates artifact retention so delayed deploy attempts can still access the rendered site bundle.

• Root cause

  • deploy depends on an artifact produced in render, but the artifact was not retained long enough for delayed/retried deploy attempts.

• Workflow change

  • Updated .github/workflows/render-book.yml in the Upload Pages artifact step to explicitly retain the artifact for 7 days.

• Behavioral impact

  • Deploy retries/reruns within the retention window can fetch the same github-pages artifact without requiring a fresh render.

- name: Upload Pages artifact
  if: github.event_name == 'push' && github.ref == 'refs/heads/main'
  uses: actions/upload-pages-artifact@v3
  with:
    path: _site
    retention-days: 7