hub v0.24.0: dispatcher routes critical severity (+ nil-prefs crash guard)

ProcessEvent routed only warning/error; a critical-severity event was silently dropped.
Now routes warning/error/critical, logs unrecognized severities, and guards a nil
GetNotificationPrefs (which would panic/crash the hub). host_disk_critical emits its
natural critical severity.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HxLA1mZurFq9kt8hneFeCs

hub/internal/monitor/host_disk_test.go

@@ -117,8 +117,8 @@ func TestHostDiskChecker_Severity(t *testing.T) {
 	dc.Check()
 	saveDiskReport(t, st, 97)
 	dc.Check()
-	if len(sev) != 2 || sev[0] != "warning" || sev[1] != "error" {
-		t.Fatalf("severities = %v, want [warning error] (critical band must be 'error' so the dispatcher routes it)", sev)
+	if len(sev) != 2 || sev[0] != "warning" || sev[1] != "critical" {
+		t.Fatalf("severities = %v, want [warning critical] (hub v0.24.0 routes the natural critical severity)", sev)
 	}
 }