feat(mcp-gateway): add NixOS module with systemd service

Provides services.mcp-gateway module for easy systemd integration:
  config.services.mcp-gateway = {
    enable = true;
    configFile = "/etc/mcp-gateway/gateway.yaml";
  };

modules/mcp-gateway.nix

@@ -0,0 +1,46 @@
+{ config, lib, pkgs, ... }:
+
+let
+  cfg = config.services.mcp-gateway;
+  package = pkgs.mcp-gateway;
+in
+
+{
+  options.services.mcp-gateway = {
+    enable = lib.mkEnableOption "MCP Gateway service";
+
+    package = lib.mkOption {
+      type = lib.types.package;
+      default = package;
+      description = "The mcp-gateway package to use.";
+    };
+
+    configFile = lib.mkOption {
+      type = lib.types.path;
+      default = "/etc/mcp-gateway/gateway.yaml";
+      description = "Path to the mcp-gateway configuration file.";
+    };
+
+    settings = lib.mkOption {
+      type = with lib.types; attrsOf (attrsOf anything);
+      default = {};
+      description = "Configuration options for the systemd service.";
+    };
+  };
+
+  config = lib.mkIf cfg.enable {
+    systemd.services.mcp-gateway = {
+      description = "MCP Gateway";
+      wantedBy = [ "multi-user.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;
+    };
+  };
+}