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), call the Skill tool with "codebase-design" and 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.

Four rules, all error:

1. Entry-point boundary: code outside a package (app code or another package) may import only that package's entry points (its root files), never anything in its subfolders.
2. Intra-package freedom: a package's own files import each other freely.
3. Tests through the entry points: files under <pkg>/tests/ may import any package's entry points and their own tests/ fixtures, but never any package's subfolder internals (not even their own). Integration tests across packages are fine; deep imports are not.
4. No cycles: no dependency cycles.