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

packages/mcp-gateway/package.nix

@@ -27,6 +27,24 @@ rustPlatform.buildRustPackage rec {
 
   doCheck = false;
 
+  postInstall = ''
+    install -Dm644 /dev/stdout $out/lib/systemd/user/mcp-gateway.service <<EOF
+[Unit]
+Description=MCP Gateway
+After=network-online.target
+Wants=network-online.target
+
+[Service]
+Type=simple
+ExecStart=%h/.config/mcp-gateway/mcp-gateway --config %h/.config/mcp-gateway/gateway.yaml serve
+Restart=on-failure
+RestartSec=5
+
+[Install]
+WantedBy=default.target
+EOF
+  '';
+
   passthru.category = "MCP Servers";
 
   meta = with lib; {