openai/gpt-chat-latest → B (4:1)

Side B adds a substantial new CLI capability for creating private rooms (`room create`), wires it through command parsing and RPC, simplifies the room model by removing unused visibility state (replacing a `HashMap` of room metadata with a `HashSet` of room IDs), and updates tests and documentation accordingly. Side A mainly corrects the thread post index semantics by enforcing the invariant with `expect`, always rendering the post link, updating documentation, and adding tests, which is a useful but narrower consistency fix.