feat: add grill skill merging domain-modeling

Combine the interview-driven grill and the domain-modeling discipline into a
single model-invokable grill skill, so other skills can reach it as a backbone.

- One question at a time, each with options plus a recommendation and rationale,
  and a context quote when it turns on existing material.
- Agent-agnostic mode selection: discover .agents/CONTEXT.md then
  .claude/CONTEXT.md, and drive Domain Modeling, Free, or Clarification.
- Domain Modeling Mode maintains the glossary inline (CONTEXT-FORMAT.md) and
  offers ADRs under the three-gate test.
- Never begin implementation without an explicit user instruction, even when a
  caller skill invoked grill.

skills/grill/SKILL.md

@@ -6,8 +6,7 @@ description: Interview the user relentlessly to stress-test a plan or design bef
 # grill
 
 Interview the user relentlessly about every aspect of a plan or design.
-Walk down each branch of the decision tree, resolving the dependencies between decisions one at a time.
-Keep going not until it feels like you understand each other, but until every branch carries an explicit decision and no dependency between decisions is left open.
+Keep going not until it feels like you understand each other, but until the decision tree is walked to exhaustion.
 
 Ask one question at a time, in plain text, and wait for the answer before the next.
 Never use an interactive question tool.
@@ -46,7 +45,7 @@ Alongside the questions, challenge and stress-test:
 - Probe edge cases with concrete scenarios that force the boundaries between concepts to be made explicit.
 - When a stated behavior contradicts what the code actually does, surface the contradiction with both sides quoted.
 
-In Domain Modeling Mode, record each term the moment it settles.
+In Domain Modeling Mode, feed the settling terms and decisions into the glossary and ADRs below.
 
 Done when every branch carries an explicit decision and no dependency between decisions is left open.
 
@@ -62,7 +61,6 @@ Done when the summary is delivered and grill is waiting.
 ## Domain Modeling Mode
 
 Active only when step 1 selected this mode.
-It keeps the project's domain model sharp as the interview surfaces and settles terms, and never touches implementation detail.
 
 ### Glossary