feat: add corosync quorum health check for proxmox-host entities

Adds pvecm_quorum_check.sh probe script and wires it into the
checkdefaults system as a new 'quorum' monitoring kind on proxmox-host
entities. Runs every 60s via ssh-script, surfaces unhealthy signal when
cluster loses quorum.

Closes the monitoring blind spot that let the 2026-08-12 3.5h corosync
flapping outage go undetected (ping passed, cluster was non-quorate).

Changes:
- seeds/ontology.yaml: proxmox-host declares monitoring: [quorum]
- internal/checkdefaults/defaults.go: KindQuorum builder
- internal/checkdefaults/build_test.go: 2 new test cases
- checks/pvecm_quorum_check.sh: new probe (deployed to hubris + strong)
- VERSION: 0.30.2 -> 0.31.0

internal/checkdefaults/defaults.go

@@ -33,6 +33,7 @@ const (
 	KindBackup     = "backup-freshness"
 	KindCertExpiry = "cert-expiry"
 	KindVMStatus   = "vm-status"
+	KindQuorum     = "quorum"
 	KindDNS        = "dns"
 )
 
@@ -360,6 +361,15 @@ func buildKind(kind string, t Target, attrs map[string]any, host, user string, p
 			config:   map[string]any{},
 			interval: 60,
 		}}, ""
+
+	case KindQuorum:
+		// Proxmox cluster quorum via `pvecm status`. Only meaningful on
+		// proxmox-host entities. Runs every 60s — corosync flaps are
+		// transient and the probe is lightweight (local binary, no network).
+		if host == "" {
+			return nil, "no address on the entity or its host"
+		}
+		return []checkDef{ssh("pvecm_quorum_check.sh")}, ""
 	}
 
 	return nil, "no builder for this kind yet"