feat(grilling): round-by-round frontier interview

Rework grilling from one-question-at-a-time to asking the whole
frontier each round, with background sub-agents for fact-finding so
research never blocks the round. Fold the batch-grill-me experiment
into grilling and delete it. Re-sync the docs page, including how to
opt back into one-at-a-time via global CLAUDE.md.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

skills/in-progress/README.md

@@ -10,4 +10,3 @@ Skills that are still being developed. They're not ready to ship — expect roug
 - **[claude-handoff](./claude-handoff/SKILL.md)** — Hand the current conversation off to a fresh background agent that picks up the work immediately, seeded with a handoff summary via `claude --bg`. User-invoked.
 - **[setup-ts-deep-modules](./setup-ts-deep-modules/SKILL.md)** — Wire dependency-cruiser into a TypeScript repo so each package is a deep module — implementation hidden in subfolders, reachable only through its entry-point files, tests exercising it through those. User-invoked.
 - **[to-questionnaire](./to-questionnaire/SKILL.md)** — Turn a decision you can't fully answer into a Markdown questionnaire for someone else to fill in async, or over a meeting. It grills you about the send (who it's for, what you need back), not the subject. User-invoked.
-- **[batch-grill-me](./batch-grill-me/SKILL.md)** — Relentless interview that walks the design tree in rounds instead of one question at a time — each round asks the whole frontier of decisions whose prerequisites are already settled, then recomputes from your answers. User-invoked.