fix(skills): add blocked grill questions

src/skills/grill/SKILL.md

@@ -32,7 +32,8 @@ A completed grill summary is not an instruction to build.
 3. Start the session.
    Present the full open-question list in short form.
    Include titles only, not full question bodies.
-   Mark each unresolved question with `- [ ]`.
+   Mark unresolved questions with `- [ ]`.
+   Mark blocked questions with `- [b]`.
    Then ask the first numbered question in the same turn.
 
    Finish when the user sees the open-question list and the first question.
@@ -67,16 +68,21 @@ When code contradicts the user's claim, quote both the claim and the code behavi
 
 ## Open-question accounting
 
-Use `- [ ]` for unresolved questions.
+Use `- [ ]` for unresolved questions that are ready to ask.
+Use `- [b]` for blocked questions with unsettled prerequisites.
 Use `- [+]` for questions added by the latest answer.
 Use `- [u]` for questions updated by the latest answer.
 Use `- [x]` for questions resolved by the latest answer.
 Use `- [-]` for questions removed by the latest answer.
 
 Show `- [u]`, `- [x]`, and `- [-]` for one turn only.
-After that turn, keep an updated question visible as `- [ ]`.
+After that turn, keep an updated question visible as `- [ ]` or `- [b]`.
 After that turn, remove resolved and removed questions from the visible list.
 
+Use `- [b]` for questions the agent knows exist but cannot ask yet.
+When a blocked question becomes askable, mark it with `- [u]` for one turn.
+Do not use `- [+]` for unblocked questions because they are not new.
+
 Add questions only when the latest answer directly causes them.
 When the user extends scope instead of answering, account for the new question before pursuing it.
 Do not add speculative questions to the open-question list.