Document conversation capture and continuation contracts

prepared_execution.go

@@ -15,6 +15,9 @@ const preparedExecutionString = "promptkit.PreparedExecution{opaque}"
 // A successful PrepareExecution may retain an Observer function in this
 // handle's private payload. The callback and other private execution settings
 // are cleared on claim completion or Discard. Details never exposes them.
+// 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.
 type PreparedExecution struct {
 	internal *usecase.PreparedExecution
 }