PR preview deploys via GitHub Actions

Build once with vercel build, deploy the prebuilt output, and comment the URL.

copyname: Preview
on: pull_request
jobs:
  deploy:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - run: npm i -g vercel@latest
      - run: vercel pull --yes --environment=preview --token=${{ secrets.VERCEL_TOKEN }}
      - run: vercel build --token=${{ secrets.VERCEL_TOKEN }}
      # ...deploy --prebuilt and post the URL (full YAML for members)

Full workflow (with the sticky PR comment + concurrency guard) included for members.