The standalone node detection in discoverClusterNodes was only checking
stderr for "not part of a cluster" messages, but some Proxmox versions
write these messages to stdout instead. This caused the fallback to
discoverLocalHostAddresses to never trigger, leaving temperature
monitoring broken on standalone nodes.
Changes:
- Check both stdout and stderr for standalone node indicators
- Document exit code 255 in addition to code 2
- Improve error logging to show both stdout and stderr
This ensures standalone nodes correctly fall back to local address
discovery regardless of where pvecm writes its error messages.
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