Add a machine
Put a host on the board without putting a key in git.
Name the env vars
Pick SSH_KEY_HOST and HEARTBEAT_TOKEN_HOST. Put the key and the token in the environment or in Actions secrets.
Add the row
In Settings, add the name, documentation-style host, and SSH user. Save the JSON and turn demo mode off.
Install the reporter
On the host, run reporter/heartbeat.mjs against a URL that is only reachable on your private network.
Confirm the badge
Wait for the status Action. Checks normal means a fresh observation. Unknown means nothing has reported.