Serialize GitHub Pages deploys to prevent in-progress deployment failures

[Copilot]

The deploy job was intermittently failing when a new Pages deployment started while another deployment was still in progress. This PR removes that race by enforcing workflow-level deployment serialization.

• Problem addressed

  • GitHub Actions deploy failed with Deployment request failed ... due to in progress deployment, causing otherwise valid pushes to fail at deploy time.

• Workflow change

  • Added workflow-level concurrency to .github/workflows/build.yml.
  • Uses a shared pages group with cancel-in-progress: false so deploys queue instead of preempting/failing.

• Behavioral impact

  • Only one Pages deployment from this workflow runs at a time.
  • Subsequent runs wait for the active deployment to complete rather than failing immediately.

concurrency:
  group: pages
  cancel-in-progress: false