Update horizon-flake-parts to 0.9.0 and consolidate configuration

Summary

Updates horizon-flake-parts from 0.8.0 to 0.9.0 and consolidates configuration files.

Changes

• Update horizon-flake-parts to 0.9.0
• Consolidate configuration: Merge ci.nix, systems.nix, and treefmt.nix into single nix/configuration.nix
• Remove direct treefmt-nix input (now provided by horizon-flake-parts)
• Use horizon.formatting.enable = true with custom excludes
• Move generated files to gen/ directory:
  • initial-packages.nix → gen/initial-packages.nix
  • pkgs/ → gen/pkgs/

Benefits

• Cleaner repository structure with consolidated configuration
• Reduced dependencies (no direct treefmt-nix input needed)
• Consistent formatting setup across all horizon projects
• Organized generated files in gen/ directory

🤖 Generated with Claude Code