feat(agent): unify service statuses across monitor impls

agent/internal/models/service.go

@@ -1,6 +1,22 @@
 package models
 
+// ServiceStatus represents the unified status of a service across all monitor types.
+type ServiceStatus string
+
+const (
+	StatusRunning  ServiceStatus = "running"
+	StatusStopped  ServiceStatus = "stopped"
+	StatusDegraded ServiceStatus = "degraded"
+	StatusPending  ServiceStatus = "pending"
+	StatusUnknown  ServiceStatus = "unknown"
+)
+
+// IsHealthy reports whether the service is stable enough for dependents to rely on.
+func (s ServiceStatus) IsHealthy() bool {
+	return s == StatusRunning
+}
+
 type Service struct {
 	Name   string
-	Status string
+	Status ServiceStatus
 }