Add setup-deep-modules skill (in-progress)

A user-invoked setup skill that wires dependency-cruiser into a
TypeScript repo so each package under the packages root is a deep
module: everything hidden behind its `index.ts` interface.

Ships a `.dependency-cruiser.cjs` template with four error-level rules —
index boundary from app code, index boundary across packages (with
intra-package freedom via $1 back-references), tests-through-the-index,
and no-circular — plus a commented layering stub. The skill installs the
tool, folds a `lint:boundaries` script into the repo's umbrella check,
scaffolds an example package, and self-verifies that a deep import fails.

Rules validated against a live dependency-cruiser 16 run.

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

skills/in-progress/README.md

@@ -8,3 +8,4 @@ Skills that are still being developed. They're not ready to ship — expect roug
 - **[writing-fragments](./writing-fragments/SKILL.md)** — Grilling session that mines you for fragments — heterogeneous nuggets of writing — and appends them to a single document as raw material for a future article.
 - **[writing-shape](./writing-shape/SKILL.md)** — Take a markdown file of raw material and shape it into an article paragraph by paragraph, arguing format choices at each step.
 - **[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-deep-modules](./setup-deep-modules/SKILL.md)** — Wire dependency-cruiser into a TypeScript repo so each package is a deep module — everything hidden behind its `index.ts`, tests exercising it through that interface. User-invoked.