Replace brittle pvecm nodes CLI parsing with pvesh API calls. The old
approach used awk field positions ($4) which breaks across Proxmox
versions, locales, or output format changes.
Added get_cluster_node_names() helper that:
- Prefers pvesh get /cluster/status --output-format json (structured)
- Falls back to pvecm nodes CLI parsing if pvesh unavailable
- Uses python3 for JSON parsing (always available on Proxmox)
Related to #738
649278bf5f