Follow a session live

A server-sent event stream. Each transcript entry is an entry event with id: <seq>; approvals, questions, waiting messages, status and usage arrive as approval, ask, pending, status and usage events, each data a SessionEvent. While the model writes, reply events carry its text as it streams (an id, an offset and the text from there; offset 0 is the whole text so far, which a stream opened mid-reply gets first): live only, never replayed, ended by the assistant entry itself. Reconnect with Last-Event-ID (or ?after=) to get every later entry exactly once. A stream that has to end says why in an error event.