feat(git): carry the commit identity on every host

Default the module on rather than restating it per host, so a new host
commits without remembering the line. A host that should not carry a
personal identity sets enable to false.

modules/git.nix

@@ -9,8 +9,17 @@ let
   user = config.user.name;
 in
 {
-  options.modules.git.enable =
-    lib.mkEnableOption "git for the primary user, carrying the operator's commit identity";
+  options.modules.git.enable = lib.mkOption {
+    type = lib.types.bool;
+    default = true;
+    description = ''
+      Whether to configure git for the primary user, carrying the operator's
+      commit identity.
+
+      On by default, so every host can commit without restating it. A host that
+      should not carry a personal commit identity sets this to false.
+    '';
+  };
 
   config = lib.mkIf cfg.enable {
     home-manager.users.${user}.programs.git = {