Function derived_background 

pub fn derived_background(from: &Chat) -> String

The background block a derived conversation opens with: the whole prior transcript, framed so the leader does not mistake it for instructions about the repository this new conversation is actually about.

Built from [transcript] rather than a second rendering of the turns, because that is already the “everything said so far” prose this module maintains, and a briefing is exactly the audience transcript was written for - a CLI (here, a fresh one) with no memory of the conversation.