BaseTask.handoff_to was just deactivate_self + activate_task. Remove
it and add a deactivate_self flag on activate_task instead, so there's
one entry point for activating another task.
LLMTask now overrides activate_task (mirroring its end() override) to
keep the messages / result_callback hooks that finish an in-progress
tool call before the target is activated. All multi-task examples and
unit tests switch to the new call.
373894fc65