READ THE SHARED SCRATCHPAD (single source of truth for the room). Read-only. Returns 'content', last 'editor', 'updated_at' and the audit 'history'. USE WHEN: right after chat_join, and immediately before EVERY chat_save_doc or before proposing a major architectural/code change in that room, so your work builds on the current state. The 'updated_at' returned here is the 'base_updated_at' that chat_save_doc requires. DO NOT: re-read repeatedly without a reason; an md_update seen in chat_messages, or an imminent write, is a reason.