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

checks/pvecm_quorum_check.sh

@@ -0,0 +1,12 @@
+#!/usr/bin/env bash
+# pvecm_quorum_check.sh — Proxmox cluster quorum status.
+# Runs on a PVE host. Fails if the node is not quorate.
+set -euo pipefail
+
+# pvecm status exit code is non-zero on non-quorate nodes
+# (e.g. "Quorate: No — Activity blocked")
+if pvecm status 2>/dev/null | grep -q 'Quorate.*Yes'; then
+  echo '{"health":"healthy","metrics":{"quorate":1}}'
+else
+  echo '{"health":"unhealthy","metrics":{"quorate":0}}'
+fi