docs: expand JSDoc for onSessionTranscriptUpdate params and return

src/sessions/transcript-events.ts

@@ -10,6 +10,9 @@ const SESSION_TRANSCRIPT_LISTENERS = new Set<SessionTranscriptListener>();
  * Register a listener for session transcript updates.
  * Returns an unsubscribe function. Listeners are guarded with try/catch
  * so a throwing subscriber cannot prevent other listeners from firing.
+ *
+ * @param listener - Callback invoked with the updated session file path.
+ * @returns Unsubscribe function; call it to remove the listener.
  */
 export function onSessionTranscriptUpdate(listener: SessionTranscriptListener): () => void {
   SESSION_TRANSCRIPT_LISTENERS.add(listener);