Function render_transcript 

pub fn render_transcript(session: &Session, format: HumanExportFormat) -> String

Render session’s conversation (session.messages, in order) for a human, in format. Pure/read-only: session is untouched, and calling this twice on the same session is idempotent (same output both times). System messages are included — they’re part of the honest record of what happened (e.g. compaction markers, context_injections blocks).