refactor: replace read-pkg with native fs.readFile + JSON.parse (#4742)

read-pkg is used in one place (@packages/utils/pkg-check.js, an
internal dev tool). Replacing it with a 3-line helper using
node:fs/promises.readFile + JSON.parse lets us drop the dep without
losing functionality. Closes #4673 as superseded.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>