Keep the outer history_offset when _run_agent drains queued follow-ups recursively so transcript persistence includes every queued turn in the chain instead of only the last one.
The transcript extraction used len(history) to find new messages, but
history includes session_meta entries that are stripped before passing
to the agent. This mismatch caused 1 message to be lost from the
transcript on every turn after the first, because the slice offset
was too high. Use the filtered history length (history_offset) returned
by _run_agent instead.
Also changed the else branch from returning all agent_messages to
returning an empty list, so compressed/shorter agent output does not
duplicate the entire history into the transcript.
