Add horizon-gen-nix validation

Summary

• Upgrades from gitlab-ci-omnix 0.5.0 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 module system usage: uses imports instead of modules in gitlab.ci configuration

Changes

1. gitlab-ci upgrade: Replace gitlab-ci-omnix 0.5.0 input 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. Module system fix: Use correct imports field in gitlab.ci configuration

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