Add OpenClaw AI Agent module

- Systemd service running OpenClaw gateway
- Configurable via sops secrets
- Runs on localhost:18789

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

modules/default.nix

@@ -5,5 +5,6 @@
     ./podman.nix
     ./nginx.nix
     ./searxng.nix
+    ./openclaw.nix
   ];
 }