Document and demonstrate execution controls

prepared_execution.go

@@ -18,6 +18,9 @@ const preparedExecutionString = "promptkit.PreparedExecution{opaque}"
 // It also retains RunRequest.CaptureExchange and RunRequest.ContinueFrom for
 // execution of the frozen snapshot. Only RunPrepared can produce its completed
 // [Exchange]; preparation alone does not.
+// RunRequest.OperationTimeout and RunRequest.OnPrepared are retained as
+// selections without an active idle timer; their execution lifecycle is
+// described on those fields.
 type PreparedExecution struct {
 	internal *usecase.PreparedExecution
 }