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)

config.example.json

@@ -4,5 +4,6 @@
   "apiKey": "replace-with-server-api-key",
   "reportIntervalSec": 30,
   "logLevel": "info",
-  "rootFs": "/host"
+  "rootFs": "/host",
+  "monitorPort": 0
 }