doc-counts: check tracked files, not the filesystem

It walked the working tree and swept in a nested git worktree left lying around, then failed on THAT checkout's copies of the docs. A checkout which is not part of this commit cannot make this commit wrong. `git ls-files '*.md'` is both narrower and exactly the intent: the docs we ship.

▶ Play it: Neon Fiscal Year → Benthic Patience → Silken Reentry — a constellation across three families — the path is the mix https://stellate.app/?genre=citypop&seed=9135&path=1530.6221,586.10667,1376.19565&xdur=0.056

38286bc https://github.com/aboard-io/stellate/commit/38286bcfa23acd43ea777f60fd046d455ebd8402