Update horizon-flake-parts to 0.12.1
Updates horizon-flake-parts to 0.12.1, which includes:
- gitlab-ci 0.10.1 with strict-flake-inputs validation
- Support for short-form branch refs (lts/, master, main)
- Automatic validation that all direct flake inputs point to protected branches or tags
Co-Authored-By: Claude noreply@anthropic.com