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";
  };

flake.nix

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