rename() to rename the current chat. It is fire-and-forget: space.send(rename(...)) resolves to undefined.
space.rename(displayName) is sugar for space.send(rename(displayName)). The builder throws at construction time if displayName is empty.
Per-platform constraints, such as iMessage requiring remote mode and a group chat, surface as an from the provider’s send action.
The content is bidirectional: on platforms that report it (iMessage remote mode today), someone else renaming the chat arrives as an inbound message with content.type === "rename" and the new displayName. message.sender is the user who renamed the chat, or undefined when the platform recorded no actor.