Add treefmt-nix integration
Summary
Adds treefmt-nix integration to the horizon-spec Haskell repository with the following formatters:
- deadnix: Remove unused Nix code
- statix: Lints and suggestions for Nix
- nixpkgs-fmt: Format Nix code
- cabal-fmt: Format Cabal files
- stylish-haskell: Format Haskell code
All code has been formatted according to these rules.