_run_post_turn_followups claimed the session for a goal continuation
checking only running, so a Stop that landed after the model answered
but before the follow-ups ran still chained the next goal turn. Refuse
it while the Stop latch is set; the goal resumes after the next prompt.
Bot live delivery stays ungated on purpose: an envelope is a person's
message rather than an automatic turn, and only a local prompt clears
the latch. A comment at the gate says so.
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