fix(grill): limit frontier wording changes

src/skills/grill/SKILL.md

@@ -7,7 +7,17 @@ Interview the user relentlessly until you reach a shared understanding. Map this
 
 Maintain the design tree in a new Markdown file. Use the file as the source of truth.
 
-Work the tree one question at a time. The **frontier** is every decision whose prerequisites are already settled: the questions you can ask now without guessing at answers you have not heard yet. Record the next frontier question in the file, then ask it and wait for the user's answer.
+Build the **frontier** before asking the first question.
+The frontier contains every unresolved decision with settled prerequisites.
+
+After each answer, update the design tree.
+Display every frontier decision.
+Mark the selected decision.
+
+Ask one selected decision each turn.
+A clarification keeps that decision open.
+Answer the clarification.
+Then ask the decision again.
 
 Format a question like so: