Update to horizon-flake-parts 0.10.0

Summary

Updates the project to use horizon-flake-parts 0.10.0, which consolidates gitlab-ci, horizon-supported-systems, and treefmt-nix into a single dependency with reusable modules.

Changes

• Replace multiple inputs with horizon-flake-parts 0.10.0:

  • Removed gitlab-ci 0.8.0 input
  • Removed horizon-supported-systems 1.1.0 input
  • Removed treefmt-nix input
  • Added horizon-flake-parts 0.10.0 input

• Centralized configuration:

  • Created nix/configuration.nix with horizon module configuration
  • Removed old configuration files:
    • nix/ci.nix (replaced by horizon.ci.enable = true)
    • nix/systems.nix (replaced by horizon.systems.supported.enable = true)
    • nix/treefmt.nix (replaced by horizon.formatting.enable = true)

• Updated flake.lock with new dependency tree

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