Set up automatic pull request preview deploys to Netlify

The way this is set up on GitHub right now is via Netlify's native integration, so we'll need to roll our own. Here's a sample pipeline that ChatGPT conjured:

# .tangled/workflows/deploy-preview.yml
when:
  - event: ["pull_request"]
    # 'branch' here is the PR's *base* branch filter (target)
    branch: ["main"]

dependencies:
  nixpkgs:
    - nodejs
    - git

steps:
  - name: Install
    command: |
      npm ci

  - name: Build (Astro)
    command: |
      npm run build

  - name: Compute preview alias
    # Create a stable alias like pr-<branch>-<shortsha>, sanitize slashes
    command: |
      BRANCH="$(git rev-parse --abbrev-ref HEAD | tr '/ _' '---')"
      SHA="$(git rev-parse --short HEAD)"
      echo "ALIAS=pr-${BRANCH}-${SHA}" >> $SPINDLE_ENV_FILE

  - name: Deploy preview (draft)
    environment:
      # Pull the alias from the previous step
      ALIAS: ${{ env.ALIAS }}
    command: |
      echo "Using alias: ${ALIAS}"
      npx netlify-cli deploy \
        --dir="dist" \
        --alias="${ALIAS}" \
        --message="PR preview ${ALIAS}" \
        --site="$NETLIFY_SITE_ID" \
        --auth="$NETLIFY_AUTH_TOKEN"