Two LLM tasks (greeter and support) handing off to each other over
the local `AsyncQueueBus`. The main task owns the transport
pipeline (STT, TTS, transport I/O) and the child tasks each run
their own LLM behind a `BusBridgeProcessor`. Each child uses
`bridged=()` so `PipelineTask` auto-wraps its pipeline with
the bus edge processors, and `transfer_to_agent` / `end_conversation`
tools demonstrate `handoff_to(...)` and `end(...)`.
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