Add horizon-gen-nix validation

Summary

• Upgrades from old structure to haskell-make-package-set based configuration
• Adds gitlab-ci 0.8.0 with modular configuration (omnix, cachix, flake modules)
• Adds horizon-gen-nix validation job to ensure generated Nix files stay up to date
• Adds missing configuration files (darwin.nix, patches.nix, modifiers.nix)
• Updates initial-packages.nix to new format
• Fixes flake.nix to use import-tree properly
• Updates to modern nixpkgs via horizon-build-packages

Changes

1. gitlab-ci upgrade: Add gitlab-ci 0.8.0 with modular structure
2. Modular structure: Use separate modules for omnix, cachix, and flake validation
3. horizon-gen-nix validation: Add horizon-gen-nix job that runs horizon-gen-nix 0.14.0 and checks for file changes
4. Package set modernization: Migrate to haskell-make-package-set structure
5. Configuration files: Add all required configuration files to match newer branches

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