Delete thread checkpoints, side-question costs, and offloaded history.
Removes the thread's checkpoint/write rows, then makes a best-effort attempt
to remove the per-thread compaction archive and handoff snapshots under
~/.deepagents (local mode) so deletion does not leave orphaned history
behind. History cleanup failures are logged, not raised, and do not affect
the return value, which reflects only whether checkpoint rows were removed.
