agent start: sweep config snapshots left by a forced termination #667

Follow-up from #648 (sandboxed launch).

tps agent start reads --config once into a private snapshot under ~/.tps/launch/config-* and removes it on normal exit and on SIGINT, SIGTERM and SIGHUP. A forced termination (for example SIGKILL, or a host crash) leaves the snapshot directory in place.

Want: at the next launch, remove ~/.tps/launch/config-* snapshot directories that no running launcher owns (for example, record the owning pid in the snapshot directory and sweep entries whose pid is not alive), with a test that leaves a stale snapshot behind and shows the next launch removes it while a live launcher's snapshot is kept.

Refs #648