Sessions name themselves

A session gets a short title of its own once there is enough of an exchange to summarize, instead of staying “Untitled session” until you rename it by hand.

• The title appears in the header as soon as it is ready, without a reload.
• It is written with your own provider key, on whichever model you have set for utility work. A workspace with no utility model keeps its sessions untitled.
• A session that already has a name is never renamed, including one you typed yourself.
• Naming runs after the reply has landed, so it never holds up an answer, and if it fails nothing else about the session is affected.