channelhealth: controller->agent channel health-check (periodic probe + classified operator alert) v0.90.0

New internal/channelhealth Checker: ~60s probe via the PRODUCTION memoized client
(Server.ProbeAgentChannel, GET /storage), classifies failures (spike Q1 map), debounces transient
reasons (N>=2; construction error latches distinctly), seeds first obs, alerts operator+dashboard on
transition. Notifier.NotifyAgentChannelDown/Recovered (English, operator-only), AlertManager dashboard
banner (Hungarian). No agent/hub change. Spike-proven.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EPZ4GJ8L5Jqf8UiPwbn1kt

controller/internal/notify/notifier.go

@@ -335,6 +335,26 @@ func (n *Notifier) NotifyStorageReconnected(label string) {
 		fmt.Sprintf("Meghajtó újra csatlakoztatva: %s", label), StorageDetails{Label: label})
 }
 
+// AgentChannelDetails carries the classified reason for a controller→agent channel-down event.
+type AgentChannelDetails struct {
+	Reason string `json:"reason"`
+}
+
+// NotifyAgentChannelDown sends an OPERATOR-facing controller→agent channel-down event (the message is
+// English — the customer can't act on "the agent re-keyed", so this never reaches them: the event type
+// is not a customer notification toggle, same as the host_* operator events). eventType/severity/
+// message come from the channelhealth classifier (spike Q1 map).
+func (n *Notifier) NotifyAgentChannelDown(reason, eventType, severity, message string) {
+	n.PushEvent(eventType, severity, message, AgentChannelDetails{Reason: reason})
+}
+
+// NotifyAgentChannelRecovered sends the recovery event (info → logged, no email, mirrors
+// storage_reconnected).
+func (n *Notifier) NotifyAgentChannelRecovered() {
+	n.PushEvent("agent_channel_recovered", "info",
+		"Controller→agent channel recovered — local-API reachable again.", nil)
+}
+
 // NotifyAppDeployed sends an app deployment event.
 func (n *Notifier) NotifyAppDeployed(stackName, displayName string) {
 	n.PushEvent("app_deployed", "info",
@@ -630,4 +650,3 @@ func statusRank(status string) int {
 		return 0
 	}
 }
-