Personal node heartbeat probe
Runs on JD-OPS-CENTER and checks one registered node through a private SSH
alias. The remote action is fixed and read-only: service state, disk usage and
uptime. A JSON receipt is written under /var/lib/guanghu/receipts/ every five
minutes.
Real addresses and SSH identity paths belong in /etc/guanghu/nodes/ssh/config
on the ops center, never in this repository.