Resolves the cluster-facing short name for this host, trying in order:
/etc/hosts lookup by a local IP (shortest alias wins);
~/.ssh/config Host entry whose Hostname is a local IP (CRLF-safe);
hostname -s.
localHostLabel()Character(1) short name, or NULL if none could be determined.
Useful for deriving the pane-title host prefix when the cluster knows this
machine by a name different from hostname -s (e.g. mega, whose raw
hostname is the node id but whose cluster alias is mega via /etc/hosts).