flake.nix: Add and configure treefmt-nix for nix fmt

flake.nix

@@ -5,6 +5,8 @@
     nixpkgs.url = "github:NixOS/nixpkgs/nixos-24.11";
     flake-parts.url = "github:hercules-ci/flake-parts";
     flake-parts.inputs.nixpkgs-lib.follows = "nixpkgs";
+    treefmt-nix.url = "github:numtide/treefmt-nix";
+    treefmt-nix.inputs.nixpkgs.follows = "nixpkgs";
     git-hooks-nix.url = "github:cachix/git-hooks.nix";
     git-hooks-nix.inputs.nixpkgs.follows = "nixpkgs";
   };
@@ -21,6 +23,7 @@
         "aarch64-linux"
       ];
       imports = [
+        inputs.treefmt-nix.flakeModule
         inputs.git-hooks-nix.flakeModule
       ];
 
@@ -43,7 +46,7 @@
 
         devShells.default = pkgs.mkShell {
           packages = with pkgs; [
-            alejandra
+            self'.formatter.outPath # Add all formatters to environment
           ];
           shellHook = ''
             ${config.pre-commit.installationScript}
@@ -51,7 +54,17 @@
         };
 
         pre-commit.settings.hooks = {
-          alejandra.enable = true;
+          treefmt.enable = true;
+        };
+
+        treefmt = {
+          programs = {
+            alejandra.enable = true;
+          };
+          settings.global.excludes = [
+            ".envrc"
+            "*.sieve"
+          ];
         };
       };