Modernize flake structure and add horizon-spec 0.16.0
Summary
This MR modernizes the repository structure to follow current best practices:
- Replace lint-utils with treefmt-nix
- Add import-tree for automatic module loading from nix/ directory
- Update gitlab-ci-omnix from 0.3.1 to 0.5.0
- Add treefmt-nix with deadnix, dhall, nixpkgs-fmt, and statix
- Add horizon-supported-systems for system configuration
- Add horizon-spec 0.16.0 to the versions list
Flake structure changes
- Simplify flake.nix to minimal import-tree-based configuration
- Create modular nix/ directory structure:
- nix/systems.nix: System configuration
- nix/ci.nix: GitLab CI/Omnix configuration
- nix/treefmt.nix: Code formatting configuration
- nix/packages.nix: Package building logic
- nix/devShells.nix: Development shell
- nix/nixos.nix: NixOS configuration
- Update .gitlab-ci.yml to use gitlab-ci 0.3.0