Set up automatic production 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.yml
when:
  - event: ["push"]
    branch: ["main"]

# Spindle will build a container with these tools available
dependencies:
  nixpkgs:
    - nodejs
    - git

steps:
  - name: Install
    command: |
      npm ci

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

  - name: Deploy to Netlify (prod)
    # Set NETLIFY_AUTH_TOKEN and NETLIFY_SITE_ID in Tangled → Repo → Settings → Secrets
    command: |
      npx netlify-cli deploy \
        --prod \
        --dir="dist" \
        --site="$NETLIFY_SITE_ID" \
        --auth="$NETLIFY_AUTH_TOKEN"