Add shell-uv2nix branch to workflow for testing

Author: kfl

.github/workflows/update-flake-lock.yaml

@@ -14,6 +14,7 @@ on:
       - 'pyproject.toml'
     branches:
       - 'main'  # Only run on main branch
+      - 'shell-uv2nix'  # Temporary: for testing before merge
 
 permissions:
   contents: write