strategies:abandonMove deletes a strategy only while it is still marked
unfinished, and says whether it deleted it, found it finished, or found
nothing. A device cleaning up after a move it quit in the middle of can no
longer delete a strategy whose move did finish and that has since been
moved or edited elsewhere; a device that lost finishMove's answer learns
its move finished. The deletion itself is the delete mutation's, shared.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>