feat: add MONITOR_PORT env var and monitorPort config field

- Added MonitorPort field to Config struct (TODO 5.1)
- Config loads from MONITOR_PORT or HF_MONITOR_PORT env vars
- Added to Dockerfile env defaults
- Updated config.example.json
- Merge function handles monitorPort from file config

Prepares Monitor for local plugin communication bridge (TODO 5.1, 5.2)

Dockerfile

@@ -15,6 +15,7 @@ ENV HF_MONITER_BACKEND_URL=https://monitor.hangman-lab.top \
     HF_MONITER_API_KEY= \
     HF_MONITER_REPORT_INTERVAL=30 \
     HF_MONITER_LOG_LEVEL=info \
-    HF_MONITER_ROOTFS=/host
+    HF_MONITER_ROOTFS=/host \
+    MONITOR_PORT=0
 
 ENTRYPOINT ["/usr/local/bin/harborforge-monitor"]