Upgrade to gitlab-ci 0.8.0
Summary
Upgrade to gitlab-ci 0.8.0 with flake validator and improved module structure
Changes
- Replace gitlab-ci-omnix 0.5.0 with gitlab-ci 0.8.0
- Update nix/ci.nix to use new modular import structure
- Import omnix, cachix, and flake validator modules explicitly
- Configure systems from horizon.systems.ci.targets
- Add flake validator (creates flake-lock and flake-show jobs)
New CI Jobs
- devour: Build packages across systems (from omnix module)
- cachix: Push to cachix on protected branches (from cachix module)
- flake-lock: Validate flake.lock is up to date (from flake module)
- flake-show: Validate nix flake show succeeds (from flake module)