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: Stress-test a plan, decision, or design through a one-question-at-a-time interview. Use when the user asks to grill a plan or uses a grill trigger phrase.
+---
+
+# grill
+
+Interview the user until the design tree has no open frontier.
+The design tree contains each decision and the decisions that depend on it.
+The frontier contains every question whose prerequisites are settled.
+
+Do not create glossary, context, or ADR files during grill.
+Do not begin implementation during or after grill without an explicit user instruction.
+A completed grill summary is not an instruction to build.
+
+## Run the interview
+
+1. Investigate first.
+   Read the conversation, files, and docs that can settle facts or reveal constraints.
+   Resolve factual questions yourself when tools can answer them.
+   Do not ask the user for facts you can find.
+
+   Finish when known facts, constraints, and unresolved decisions are visible.
+
+2. Build the first frontier.
+   Map the design tree from the known plan or idea.
+   Put only questions with settled prerequisites in the frontier.
+   Defer questions that depend on another open question.
+
+   Finish when each frontier question can receive an answer without guessing another answer.
+
+3. Start the session.
+   Present the known frontier in short form.
+   Include titles only, not full question bodies.
+   Then start the interview in the same turn with the first numbered question.
+
+   Finish when the user sees the known frontier and the first question.
+
+4. Ask one question at a time.
+   End each active grill turn with exactly one next numbered question.
+   Give concrete options with trade-offs.
+   Give a recommendation and rationale.
+   Wait for the user's answer before the next question.
+
+   Finish when the user has answered the current question.
+
+5. Recompute the frontier.
+   Mark the answered decision as settled.
+   Add newly unblocked questions to the frontier.
+   Remove questions that the answer made irrelevant.
+   Resolve newly answerable facts yourself before asking again.
+
+   Finish when the frontier reflects the current design tree.
+
+6. Challenge weak decisions.
+   Probe vague words, overloaded terms, and hidden assumptions.
+   Use concrete scenarios to test concept boundaries and edge cases.
+   When code contradicts the user's claim, quote both the claim and the code behavior.
+
+   Finish when the next frontier question accounts for the challenge.
+
+7. Close the interview.
+   Stop when the frontier is empty.
+   Output a concise summary of every decision reached.
+   Do not act on the summary unless the user gives a new implementation instruction.
+
+   Finish when the summary is delivered and grill waits.
+
+## Turn endings
+
+While grill is active, every assistant turn must end with exactly one of these items:
+
+- the next numbered question
+- the final decision summary
+- a plain statement that grill is paused because the user changed tasks
+
+If the turn includes a correction, file edit, apology, or explanation, still end with the next numbered question.