Snooze a running thread until its run ends

[jakeleventhal]

Snooze today is a duration. When a thread is running, the usual intent is "tell me when the agent is done," which means guessing how long the run will take.

Add an Until done option for a thread that has a run in progress. The thread leaves the inbox and comes back when that run ends, whether it completes, fails, or is stopped. A later run should not hide it again. Threads with no run in progress keep the existing time-based snooze.

#13196 implements this and was closed pending agreement on the direction and scope. The need to wait for a run without guessing a duration is the part worth deciding.