openai/gpt-chat-latest → A (4:1)

Side A replaces the pervasive `CanonicalItemUrl` string wrapper with a structural `ItemId` type, introduces dedicated `item_id`/`item_wire` modules, and propagates the new identity model through reducers, routing, ranking, APIs, and tests, creating a more durable foundation for item identity and URL handling. Side B fixes a real multi-user bug by removing shared browser sentinel delegates and updating session/auth flows and tests, but its impact is narrower than the cross-cutting architectural improvement in Side A.