Fix CI: remove checkout step, runner can't resolve gitea hostname

The build happens on the staging server via SSH, not in the runner
container, so checkout is unnecessary.

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

.gitea/workflows/deploy-staging.yml

@@ -8,8 +8,6 @@ jobs:
   build-and-deploy:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
-
       - name: Build, push, and deploy to staging
         uses: appleboy/ssh-action@v1
         with: