Add production environment to Fly deploy workflow

Links deploys to GitHub's Deployments tab with the site URL.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.github/workflows/fly.yml

@@ -7,7 +7,10 @@ jobs:
   deploy:
     name: Deploy app
     runs-on: ubuntu-latest
-    concurrency: deploy-group    # optional: ensure only one action runs at a time
+    concurrency: deploy-group
+    environment:
+      name: production
+      url: https://kennethreitz.org
     steps:
       - uses: actions/checkout@v4
       - uses: superfly/flyctl-actions/setup-flyctl@master