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

modules/mcp-gateway.nix

@@ -1,46 +0,0 @@
-{ config, lib, pkgs, ... }:
-
-let
-  cfg = config.services.mcp-gateway;
-  package = pkgs.mcp-gateway;
-in
-
-{
-  options.services.mcp-gateway = {
-    enable = lib.mkEnableOption "MCP Gateway user service";
-
-    package = lib.mkOption {
-      type = lib.types.package;
-      default = package;
-      description = "The mcp-gateway package to use.";
-    };
-
-    configFile = lib.mkOption {
-      type = lib.types.str;
-      default = "%h/.config/mcp-gateway/gateway.yaml";
-      description = "Path to the mcp-gateway configuration file.";
-    };
-
-    settings = lib.mkOption {
-      type = with lib.types; attrsOf (attrsOf anything);
-      default = {};
-      description = "Extra systemd service configuration options.";
-    };
-  };
-
-  config = lib.mkIf cfg.enable {
-    systemd.user.services.mcp-gateway = {
-      description = "MCP Gateway";
-      wantedBy = [ "default.target" ];
-      wants = [ "network-online.target" ];
-      after = [ "network-online.target" ];
-
-      serviceConfig = {
-        Type = "simple";
-        ExecStart = "${cfg.package}/bin/mcp-gateway --config ${cfg.configFile} serve";
-        Restart = "on-failure";
-        RestartSec = 5;
-      } // cfg.settings;
-    };
-  };
-}