feat: add Go-based HarborForge monitor client

- collect CPU, memory, disk, swap, load, and uptime telemetry
- detect nginx and list /etc/nginx/sites-enabled entries
- send heartbeat-v2 payload with API key auth
- provide install script, config example, and systemd unit

config.example.json

@@ -0,0 +1,7 @@
+{
+  "backendUrl": "https://monitor.hangman-lab.top",
+  "identifier": "vps-01",
+  "apiKey": "replace-with-server-api-key",
+  "reportIntervalSec": 30,
+  "logLevel": "info"
+}