Add default-instance check

- validate implicit default instance wires systemd unit name
- run new check in garnix

Tests: not run (not requested)

flake.nix

@@ -61,6 +61,7 @@
           config-options = pkgs.callPackage ./nix/checks/openclaw-config-options.nix {
             sourceInfo = sourceInfoStable;
           };
+          default-instance = pkgs.callPackage ./nix/checks/openclaw-default-instance.nix {};
           hm-activation = import ./nix/checks/openclaw-hm-activation.nix {
             inherit pkgs home-manager;
           };

garnix.yaml

@@ -11,5 +11,6 @@ builds:
     - "checks.x86_64-linux.gateway"
     - "checks.x86_64-linux.gateway-tests"
     - "checks.x86_64-linux.config-options"
+    - "checks.x86_64-linux.default-instance"
     - "checks.x86_64-linux.config-validity"
     - "checks.x86_64-linux.package-contents"

nix/checks/openclaw-default-instance.nix

@@ -0,0 +1,86 @@
+{ lib, pkgs, stdenv }:
+
+let
+  stubModule = { lib, ... }: {
+    options = {
+      assertions = lib.mkOption {
+        type = lib.types.listOf lib.types.attrs;
+        default = [];
+      };
+
+      home.homeDirectory = lib.mkOption {
+        type = lib.types.str;
+        default = "/tmp";
+      };
+
+      home.packages = lib.mkOption {
+        type = lib.types.listOf lib.types.anything;
+        default = [];
+      };
+
+      home.file = lib.mkOption {
+        type = lib.types.attrs;
+        default = {};
+      };
+
+      home.activation = lib.mkOption {
+        type = lib.types.attrs;
+        default = {};
+      };
+
+      launchd.agents = lib.mkOption {
+        type = lib.types.attrs;
+        default = {};
+      };
+
+      systemd.user.services = lib.mkOption {
+        type = lib.types.attrs;
+        default = {};
+      };
+
+      programs.git.enable = lib.mkOption {
+        type = lib.types.bool;
+        default = false;
+      };
+
+      lib = lib.mkOption {
+        type = lib.types.attrs;
+        default = {};
+      };
+    };
+  };
+
+  eval = lib.evalModules {
+    modules = [
+      stubModule
+      ../modules/home-manager/openclaw.nix
+      ({ lib, ... }: {
+        config = {
+          home.homeDirectory = "/tmp";
+          programs.git.enable = false;
+          lib.file.mkOutOfStoreSymlink = path: path;
+          programs.openclaw = {
+            enable = true;
+            launchd.enable = false;
+            systemd.enable = true;
+          };
+        };
+      })
+    ];
+    specialArgs = { inherit pkgs; };
+  };
+
+  hasUnit = builtins.hasAttr "openclaw-gateway" eval.config.systemd.user.services;
+  check = if hasUnit then "ok" else throw "Default Openclaw instance missing systemd.unitName.";
+  checkKey = builtins.deepSeq check "ok";
+
+in
+stdenv.mkDerivation {
+  pname = "openclaw-default-instance";
+  version = "1";
+  dontUnpack = true;
+  env = {
+    OPENCLAW_DEFAULT_INSTANCE = checkKey;
+  };
+  installPhase = "${../scripts/empty-install.sh}";
+}