refactor(registry): use xor for focus targets

configs/nixos/registry/keybind.nix

@@ -12,13 +12,9 @@
           entry:
           let
             action = entry.value.action;
-            focusTargets = lib.filter (target: target != null) [
-              action.direction
-              action.workspace
-            ];
           in
           (action.kind == "exec" && action.command != null)
-          || (action.kind == "focus" && builtins.length focusTargets == 1)
+          || (action.kind == "focus" && lib.xor (action.direction != null) (action.workspace != null))
         ) entries;
       message = "Desktop keybind actions must set the fields their kind requires.";
     }