feat(probe): port monitor-probe collector into agent, persist latest snapshot (item 7 B1)

internal/agent/agent.go

@@ -118,9 +118,11 @@ func (e *statusError) Error() string {
 // Register registers the agent with the master
 func (a *Agent) Register() error {
 	url := fmt.Sprintf("%s/panel/api/agent/register", a.config.MasterURL)
-	payload := map[string]string{
+	payload := map[string]any{
 		"token": a.config.Token,
 		"name":  a.config.NodeName,
+		// Probe facts ride along (B1): old masters ignore the extra field.
+		"probeFacts": collectFacts(),
 	}
 	body, _ := json.Marshal(payload)
 	req, err := http.NewRequest("POST", url, bytes.NewReader(body))
@@ -280,6 +282,12 @@ func (a *Agent) sendHeartbeat() error {
 		"token":     a.config.Token,
 		"timestamp": time.Now().Unix(),
 		"telemetry": a.collectTelemetry(),
+		// Probe snapshot (B1): facts are process-cached so re-sending them
+		// costs nothing and heals a master that missed the register-time
+		// copy; metrics are sampled fresh each heartbeat. Old masters
+		// ignore both fields.
+		"probeFacts":   collectFacts(),
+		"probeMetrics": collectMetrics(),
 	}
 	if rep := a.currentUpdateReport(); rep != nil {
 		payload["updateStatus"] = rep