Keep threads already written on the AI writing tab

Makes the threads already written as free previews for up to ten contacts those contacts' threads on the sequence's AI writing tab, word for word: nothing is written again, nothing is charged and no preview allowance is spent, so each contact's thread there shows the emails the previews showed and Looks good can be given on them. A step with no current preview shows as not written. The contacts join the sequence's open writing check; one already in it with the same words keeps its verdict. This never writes: the start route is what writes a thread. The answer names any kept thread written before the plain-words check that uses banned jargon or copies the business context, so it can be written again.