fix(mcp-gateway): use nix store binary path in systemd ExecStart

Expand $out in the heredoc so ExecStart points to the actual binary
in the nix store, not %h/.config/mcp-gateway/mcp-gateway.

packages/mcp-gateway/package.nix

@@ -29,7 +29,7 @@ rustPlatform.buildRustPackage rec {
 
   postInstall = ''
     mkdir -p $out/lib/systemd/user
-    cat > $out/lib/systemd/user/mcp-gateway.service << 'EOF'
+    cat > $out/lib/systemd/user/mcp-gateway.service << EOF
 [Unit]
 Description=MCP Gateway
 After=network-online.target
@@ -37,7 +37,7 @@ Wants=network-online.target
 
 [Service]
 Type=simple
-ExecStart=%h/.config/mcp-gateway/mcp-gateway --config %h/.config/mcp-gateway/gateway.yaml serve
+ExecStart=$out/bin/mcp-gateway --config %h/.config/mcp-gateway/gateway.yaml serve
 Restart=on-failure
 RestartSec=5