background_output auto-enabled full_session when the task was still
running, returning the entire session transcript on every poll. When
the parent agent had no other work and polled in a tight loop, this
caused massive token waste because each response dumped thousands of
tokens into the conversation history.
Default full_session to false so running-task checks return a compact
status table (~200 tokens). Callers can still pass full_session=true
explicitly when they need the full transcript.
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