Add /grill-me planning interview skill

[nsmiller2501]

Summary

This PR adds /grill-me, a small planning/interview skill for stress-testing a plan or design before implementation.

What changed

• Adds the /grill-me skill.
• Adds internal and public README documentation.
• Adds /grill-me to the visible skills index.
• Preserves attribution to Matt Pocock; the skill body is taken verbatim from his grill-me skill, with documentation added here for this collection.

Why

The skill is useful as a lightweight pre-implementation forcing function: it asks one question at a time, gives a recommended answer, and keeps going until the decision tree is resolved.

Testing

• Reviewed the branch diff against the PR notes.
• No automated tests were run; this is a small skill/documentation addition.