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/web/agent_disk_handlers.go

@@ -1,6 +1,7 @@
 package web
 
 import (
+	"context"
 	"encoding/json"
 	"errors"
 	"net/http"
@@ -59,6 +60,22 @@ func (s *Server) agentClient() (*agentapi.Client, error) {
 	return s.agentCli, s.agentCliErr
 }
 
+// ProbeAgentChannel runs one controller→agent channel health probe using the PRODUCTION memoized
+// client (NOT a fresh one — spike SPIKE-controller-agent-channel-health-2026-06-29: it self-heals,
+// reflects exactly what the disk UI sees, and avoids the per-call transport leak the singleton fixed).
+// It returns whether the failure was a CONSTRUCTION error (agentClient() couldn't build — a latching
+// config fault, distinct from a runtime channel failure) and the error (nil = channel up). The probe
+// is GET /storage (cheap, read-only — the same call probeLocalAPI uses at startup). This is the
+// channelhealth.Probe seam.
+func (s *Server) ProbeAgentChannel(ctx context.Context) (constructionErr bool, err error) {
+	client, cerr := s.agentClient()
+	if cerr != nil {
+		return true, cerr
+	}
+	_, serr := client.Storage(ctx)
+	return false, serr
+}
+
 // writeDiskJSON writes the standard {ok,data,error} envelope used by the disk API.
 func writeDiskJSON(w http.ResponseWriter, status int, ok bool, errMsg string, data interface{}) {
 	w.Header().Set("Content-Type", "application/json")