Setup TS Deep Modules

Make every package in this repo a deep module: a lot of behaviour behind a small interface. A package's public surface is its entry points — the files at the package root — and everything in its subfolders is hidden. This skill installs dependency-cruiser and the rules that make the entry points the only way in, then proves the rules bite.

For the vocabulary (deep module, interface, seam, depth), run the /codebase-design skill — use its language throughout.

The shape this enforces

src/packages/
  <name>/
    index.ts        ← an entry point (public). Import this from outside.
    client.ts       ← another entry point. Packages may expose SEVERAL.
    lib/            ← implementation: hidden from outside, free to import each other.
    tests/          ← co-located tests + fixtures (a subfolder, so private).

The public surface is the package's root files — not one designated index.ts. By convention implementation lives in lib/ and tests in tests/, giving every package the same two-folder shape. The rule itself is general, though: anything in any subfolder is private, so you never extend the config to add a folder.