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.
Adapts the pipecat-subagents `examples/README.md` to the new
layout (`multi-task/` umbrella, `local-handoff/`, `distributed-handoff/`,
`remote-proxy-assistant/`, `parallel-debate/`, `code-assistant/`),
updates the agent→task / job-RPC vocabulary, drops the
single-agent and llm-and-flows examples (gone in the port), and
adds a new section for the PGMQ handoff transport.