prepare_turn(
self,
*,
model_label: str | None = None,
) -> NoneArm a deferred clear for a new turn without touching the panel yet.
The visible fan-out persists across turns; it is cleared lazily when the
next workflow actually starts a subagent (see _handle_start), so a turn
that spawns none leaves the previous results on screen. Refreshes the
session model used to label rows.