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

seeds/ontology.yaml

@@ -232,6 +232,7 @@ entity_types:
     layer: infrastructure
     lifecycle: infrastructure
     description: Machine running Proxmox VE.
+    monitoring: [quorum]  # corosync quorum check via pvecm status
     attributes:
       type: object
       properties: {pve_version: {type: string}}