Update horizon-flake-parts to 0.9.0

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 for formatting configuration
• Inherits deadnix, dhall, nixpkgs-fmt, and statix formatters from horizon-flake-parts

Benefits

• Cleaner repository structure with consolidated configuration
• Reduced dependencies (no direct treefmt-nix input needed)
• Consistent formatting setup across all horizon projects
• Centralized management through horizon-flake-parts

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