Add a [Unit] section to hyperhive-limits.conf (the drop-in written by
write_resource_limits) with:
ConditionPathIsDirectory=/run/hyperhive/agents/<name>
When this condition is not met, systemd skips the unit with result
"condition" — NOT a failure, so the start-limit counter is not
incremented. Belt-and-braces on top of the tmpfiles.d fix (subtask 2):
if a dir is somehow absent at start time, the container idles instead of
restart-looping into start-limit-hit.
Also promote AGENT_RUNTIME_ROOT to a module-level const (was duplicated
inside two functions) and remove the duplicates.
14c2c6d4a5