chore: fix formatting and add treefmt

outputs.nix

@@ -5,12 +5,14 @@
   nixpkgs,
   agenix,
   devshell,
+  treefmt-nix,
   ...
 }@inputs:
 (flake-utils.lib.eachDefaultSystem (
   system:
   let
     pkgs = import nixpkgs { inherit system; };
+    treefmtEval = treefmt-nix.lib.evalModule pkgs ./treefmt.nix;
   in
   {
     devShells.default =
@@ -26,7 +28,7 @@
       in
       import ./devshell.nix { inherit pkgs extraModules; };
 
-    formatter = pkgs.nixfmt-tree;
+    formatter = treefmtEval.config.build.wrapper;
 
     packages = import ./pkgs { inherit pkgs; } // {
       installer-iso = self.nixosConfigurations.installer.config.system.build.isoImage;
@@ -34,7 +36,6 @@
   }
 ))
 // {
-
   hmModules = import ./home/modules.nix { lib = nixpkgs.lib; };
 
   nixosConfigurations = import ./nixos/configurations.nix inputs;