Fix GitHub Pages deploy job by replacing built-in workflow with custom one

[Copilot]

The built-in pages-build-deployment dynamic workflow was failing: on the first attempt the deployment stayed stuck in deployment_queued for the full 10-minute timeout; on the re-run, only the deploy job was retried, so the github-pages artifact uploaded by build in attempt 1 had expired, causing an immediate failure.

Changes

• Added .github/workflows/deploy.yml — a self-contained Pages deployment workflow that runs checkout, artifact upload, and deploy in a single job, so the artifact is always fresh when the deploy step executes. Eliminates the split build/deploy job architecture that makes partial reruns fragile.

jobs:
  deploy:
    environment:
      name: github-pages
      url: ${{ steps.deployment.outputs.page_url }}
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: actions/configure-pages@v5
      - uses: actions/upload-pages-artifact@v3
        with:
          path: '.'
      - uses: actions/deploy-pages@v4

Concurrency group "pages" with cancel-in-progress: false prevents overlapping deployments. OIDC permissions (id-token: write, pages: write) match what deploy-pages requires.