feat: add POST /openclaw endpoint and enrich heartbeats with OpenClaw metadata

- Bridge server now accepts POST /openclaw from OpenClaw plugin
- OpenClawMeta struct stores version, plugin_version, and agents
- Heartbeat sendOnce() enriches payload with plugin metadata when available
- Telemetry Payload adds optional openclaw_version field
- README updated to document /openclaw endpoint and metadata enrichment
- All communication remains optional — Monitor functions without plugin data

internal/telemetry/telemetry.go

@@ -35,6 +35,10 @@ type Payload struct {
 	SwapPct        float64   `json:"swap_pct,omitempty"`
 	LoadAvg        []float64 `json:"load_avg,omitempty"`
 	UptimeSeconds  uint64    `json:"uptime_seconds,omitempty"`
+
+	// Optional OpenClaw metadata, enriched from plugin bridge.
+	// These fields are omitted if no plugin data is available.
+	OpenClawVersion string `json:"openclaw_version,omitempty"`
 }
 
 func BuildPayload(ctx context.Context, cfg config.Config) (Payload, error) {