Imp extension adapter that asks the model to plan, then answer.
This is NOT a DSPy adapter. It prepends a :plan output field to the
signature and delegates rendering and parsing to Imp.Adapter.Chat, so the
model produces a short plan before the final fields. It was previously named
Imp.Adapter.TwoStep; that name now belongs to the faithful port of DSPy
3.2.1 TwoStepAdapter (dee-qt5r), and this extension keeps its behavior
under an honest Imp name.