Return the recent stream-tail entries for session_id.

Args: session_id: Bernstein session whose tail to read. request: FastAPI request - used to resolve the workdir and the tail query argument.

Returns: JSON envelope with session_id plus a tail list of {ts, surface, text} entries in chronological order. An empty list signals "buffer not initialised yet" rather than an error so the polling page renders a blank pane while it waits.