Update to horizon-flake-parts 0.10.0

Summary

Modernizes the flake configuration to use horizon-flake-parts 0.10.0, which provides reusable modules for CI, systems, and formatting. This replaces the old flake-to-gitlab-ci pattern with the new modular approach while preserving all existing functionality.

Changes

• Add horizon-flake-parts 0.10.0: Provides integrated CI, systems, and formatting modules

• Modular configuration structure:

  • Switch to import-tree pattern for nix/ directory
  • Create nix/configuration.nix with horizon module configuration
  • Move perSystem configuration to nix/horizon-gen-nix.nix
  • Remove nix/default.nix (replaced by modular structure)

• GitLab CI modernization:

  • Update .gitlab-ci.yml to use new gitlab-ci 0.9.0 remote include
  • Remove old flake-to-gitlab-ci 0.3.0.0 pattern
  • Preserve impure-tests and cachix jobs

• Systems configuration:

  • Replace hardcoded systems list with horizon.systems.supported.enable
  • Automatic system detection for all supported platforms

• Formatting checks:

  • Add horizon.formatting.enable for treefmt-based formatting
  • Preserve existing lint-utils checks (cabal-fmt, dhall-format, hlint, nixpkgs-fmt, stylish-haskell, werror)

• Updated flake.lock with new dependency tree

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