docs: settle item 165 on a stable draft id, discarded at send

The item recorded three positions on what a draft's identity is and
needed a decision before it could be built. The user chose the second: a
stable id across a draft's revisions, minted fresh for the sent copy, so
the sent message is a different item from the draft. The draft is deleted
on a successful send, which is what the code already does, and the sent
message carries no draft tag.

Three things the decision describes were measured rather than assumed,
and all three are already built: DraftStore::write() removes the previous
revision, ComposeWindow removes the draft on a successful send, and the
sent copy is built fresh so it never carries the tag. The draft tag the
user saw on a Sent row came from orphan revisions threading into the
conversation, not from the sent message.

So the id is the whole remaining defect, and the mechanism is the server
rather than the local files. Measured on real mail: four revisions of one
reply, four distinct Message-IDs, every file carrying a ,U= infix. mbsync
uploads each revision before the next save removes it locally, so the
server holds four independent messages and syncs them all back. The local
cleanup is correct and cannot help, because deleting a local file does
not retract an uploaded one.

Reclassified from enhancement to defect and sized S. The entry said this
was invisible while the file is replaced correctly; the file is replaced
correctly and four messages exist anyway.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Jq9gXquUo9W4KXDagJXMmn

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