The reaper was being SIGTERM'd mid-rm because `start_new_session=True`
only forks a new POSIX session — it does not escape decnet-agent.service's
cgroup. When the reaper ran `systemctl stop decnet-agent`, systemd
tore down the whole cgroup (reaper included) before `rm -rf /opt/decnet*`
finished, leaving the install on disk.
Spawn the reaper via `systemd-run --collect --unit decnet-reaper-<pid>`
so it runs in a fresh transient scope, outside the agent unit. Falls
back to bare Popen for non-systemd hosts.