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.
Two transports of the same shape: a main task that hosts the
voice pipeline plus a network-backed `TaskBus` (`RedisBus` or
`PgmqBus`), and a standalone `llm.py` worker process for the
greeter / support LLM. Workers connect to the same bus channel,
register on the shared `TaskRegistry`, and the main task waits
on `runner.registry.watch("greeter", ...)` before sending the
welcome activation so it doesn't fire before the worker is up.
