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

flake.lock

@@ -116,7 +116,28 @@
       "inputs": {
         "flake-parts": "flake-parts",
         "git-hooks-nix": "git-hooks-nix",
-        "nixpkgs": "nixpkgs"
+        "nixpkgs": "nixpkgs",
+        "treefmt-nix": "treefmt-nix"
+      }
+    },
+    "treefmt-nix": {
+      "inputs": {
+        "nixpkgs": [
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1733440889,
+        "narHash": "sha256-qKL3vjO+IXFQ0nTinFDqNq/sbbnnS5bMI1y0xX215fU=",
+        "owner": "numtide",
+        "repo": "treefmt-nix",
+        "rev": "50862ba6a8a0255b87377b9d2d4565e96f29b410",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "treefmt-nix",
+        "type": "github"
       }
     }
   },

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}
@@ -51,8 +54,18 @@
         };
 
         pre-commit.settings.hooks = {
+          treefmt.enable = true;
+        };
+
+        treefmt = {
+          programs = {
             alejandra.enable = true;
           };
+          settings.global.excludes = [
+            ".envrc"
+            "*.sieve"
+          ];
+        };
       };
 
       flake.flakeModules = rec {