feat: implement HarborForge Monitor OpenClaw Plugin

Architecture:
- openclaw.plugin.json: Plugin manifest with config schema
- index.mjs: Plugin entry, lifecycle hooks (gateway:start/stop)
- sidecar/server.mjs: Independent Node process for telemetry

Features:
- Collects system metrics (CPU, memory, disk, load, uptime)
- Collects OpenClaw status (version, agents)
- HTTP heartbeat to HarborForge Monitor
- Config via ~/.openclaw/openclaw.json
- Sidecar auto-starts/stops with Gateway

Config options:
- enabled, backendUrl, identifier
- challengeUuid (required, from Monitor registration)
- reportIntervalSec, httpFallbackIntervalSec
- logLevel

Provides tool: harborforge_monitor_status

openclaw.plugin.json

@@ -0,0 +1,48 @@
+{
+  "id": "harborforge-monitor",
+  "name": "HarborForge Monitor",
+  "version": "0.1.0",
+  "description": "Server monitoring plugin for HarborForge - streams telemetry to Monitor",
+  "entry": "./index.mjs",
+  "configSchema": {
+    "type": "object",
+    "additionalProperties": false,
+    "properties": {
+      "enabled": { 
+        "type": "boolean", 
+        "default": true,
+        "description": "Enable the monitor plugin"
+      },
+      "backendUrl": { 
+        "type": "string", 
+        "default": "https://monitor.hangman-lab.top",
+        "description": "HarborForge Monitor backend URL"
+      },
+      "identifier": { 
+        "type": "string",
+        "description": "Server identifier (auto-detected from hostname if not set)"
+      },
+      "challengeUuid": { 
+        "type": "string",
+        "description": "Registration challenge UUID from Monitor"
+      },
+      "reportIntervalSec": { 
+        "type": "number", 
+        "default": 30,
+        "description": "How often to report metrics (seconds)"
+      },
+      "httpFallbackIntervalSec": { 
+        "type": "number", 
+        "default": 60,
+        "description": "HTTP heartbeat interval when WS unavailable"
+      },
+      "logLevel": { 
+        "type": "string", 
+        "enum": ["debug", "info", "warn", "error"], 
+        "default": "info",
+        "description": "Logging level"
+      }
+    },
+    "required": ["challengeUuid"]
+  }
+}