internal/chatarchive: batches a channel's messages (per N or T) and mints them as a 'Chat archive' forum thread through zit forum post/reply (zit#76) — one thread per channel, each batch a reply carrying the transcript, so history clones with the repo. Minting runs only on the lease holder; non-holders drop their buffers (the holder's archive folds back to every node). zitcli gains ForumPost/ForumReply/ TechnoteAdd. --chat-archive on by default. Verified live: chat batches materialize as forum threads and reply to the same thread. check-in: bd91af3158 user: dmestas tags: trunk