docs(bin): give every bin skill a README

The bin skills were the only plugin without per-skill READMEs, so a reader
had to open SKILL.md — an agent-facing contract, not an explainer — to learn
what a skill does and when it fires. Each README states purpose, triggers and
boundaries for a human audience, leaving SKILL.md free to stay terse.

plugins/bin/.apm/skills/grill-me/README.md

@@ -0,0 +1,27 @@
+# grill-me
+
+Interview the user relentlessly about a plan or design until the decision tree is fully resolved.
+
+## What it does
+
+Turns the agent into an interviewer rather than an implementer. It walks the design tree branch by branch, resolving dependencies between decisions one at a time, and offers its own recommended answer alongside each question so the user has something concrete to push against. Two rules give it its shape: **one question at a time**, and **never ask what the codebase can answer** — if a question is settleable by reading the code, the agent goes and reads the code instead of spending the user's attention on it.
+
+## Composition
+
+This is the plain grilling loop, with no documentation side effects. The sibling `grill-with-docs` skill runs the same interview but additionally challenges answers against the project's `CONTEXT.md` glossary and existing ADRs, and writes decisions back into those files as they crystallise. Reach for that one when the project has a domain model worth defending; reach for this one when it does not, or when nothing should be written down yet.
+
+`triage` composes the documented variant, not this one, when an issue needs fleshing out.
+
+## Usage
+
+```text
+/grill-me
+```
+
+Describe the plan or design to be stress-tested. Expect questions one at a time, each with a recommended answer.
+
+## Files
+
+| File | Purpose |
+|------|---------|
+| `SKILL.md` | The whole skill — the interview instruction, the one-question-at-a-time rule, and the explore-instead-of-asking rule |