add nix fmt pre-commit hook

modules/shell/git.nix

@@ -1,12 +1,22 @@
 #
 # Git
 #
-{ lib, user, ... }:
-
 {
+  lib,
+  pkgs,
+  user,
+  ...
+}: {
   programs = {
     git = {
       enable = true;
+      hooks.pre-commit = pkgs.writeShellScript "pre-commit" ''
+        nix fmt .
+        git diff --exit-code || {
+          echo "Formatter changed files — review with 'git diff', then re-stage and commit."
+          exit 1
+        }
+      '';
       settings = {
         user.name = "Kabbone";
         user.email = "tobias@opel-online.de";