feat(mcp-gateway): add systemd user unit file to package

Install mcp-gateway.service to $out/lib/systemd/user/ so it can be
activated via systemd.user.packages. The service runs:
  mcp-gateway --config ~/.config/mcp-gateway/gateway.yaml serve

flake.nix

@@ -55,10 +55,6 @@
     // {
       packages = inputs.nixpkgs.lib.recursiveUpdate blueprintOutputs.packages extraPackages;
 
-      nixosModules = {
-        mcp-gateway = ./modules/mcp-gateway.nix;
-      };
-
       overlays = {
         default = import ./overlays {
           inherit (blueprintOutputs) packages;