Add horizon-gen-nix validation

Summary

• Upgrades from gitlab-omnix-ci to gitlab-ci 0.8.0
• Migrates to modular gitlab-ci configuration with separate omnix, cachix, and flake modules
• Adds horizon-gen-nix validation job to ensure generated Nix files stay up to date
• Fixes flake.nix structure: removes duplicate code and uses import-tree properly
• Moves haskell packageSet configuration from flake.nix to nix/default.nix
• Creates nix/ci.nix for gitlab-ci configuration
• Uses correct imports field in gitlab.ci configuration

Changes

1. gitlab-ci upgrade: Replace gitlab-omnix-ci with gitlab-ci 0.8.0
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. flake.nix cleanup: Remove duplicate code and properly use import-tree
5. Module organization: Move configuration into nix/ directory for better organization

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