Each tick lists skipped items with terminal and reason (concurrency n/m with
the holding runs, serialize group holder, busy_process not started by agentd,
relay down, no route for assignee, start failed) in heartbeat.json "skipped".
A reason is logged (agentd.log / cron.log) only when it is new for that item;
the last set is kept in the state file as "skips".
jj.wang