feat(skills): add grill skill

.pi/skills/grill

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+../../src/skills/grill

src/skills/grill/SKILL.md

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+---
+name: grill
+description: Grill the user relentlessly about a plan, decision, or idea. Use when the user wants to stress-test their thinking, or uses any 'grill' trigger phrases.
+---
+
+Interview the user relentlessly until you reach a shared understanding.
+Map each decision as a **design tree**.
+
+Create a new tracked Markdown design record for every session.
+Infer its path and file name from the context.
+Report the path after creating the record.
+
+Use this record structure:
+
+```markdown
+# <topic>
+
+## Decisions
+
+### Question 1: <question>
+
+**Options**
+
+1. <option>
+2. <recommended option>
+
+**Decision:** <chosen option>
+
+## Frontier
+
+### Question 2: <question>
+
+**Options**
+
+1. <option>
+2. <recommended option>
+
+## Non-frontier
+
+### Question 3: <question>
+
+**Options**
+
+1. <option>
+2. <recommended option>
+
+**Blocked by:** <unresolved decision>
+```
+
+The **frontier** contains questions whose prerequisites are settled.
+The **non-frontier** contains questions with unresolved prerequisites.
+Record dependencies only when they help order questions.
+
+Investigate environment facts with available tools before asking a dependent question.
+Ask the user only for decisions.
+
+Ask one frontier question at a time.
+Write its question and options in the record before asking it.
+Include the recommended answer as an explicit option.
+Word the question so "yes" selects that option.
+
+After each answer, move the question to **Decisions**.
+Record the selected option.
+Recompute the design tree.
+Update the frontier and non-frontier before asking the next question.
+
+The session ends when both question lists are empty.
+Present the completed record.
+Do not act until the user confirms the shared understanding.