Use pnpm `shell-emulator` instead of `cross-env`

Editor assessment

Tooling swaps belong under `chore`, and this header says exactly what changed: pnpm's built-in `shell-emulator` replaces `cross-env`. The subject is imperative, specific, quotes the relevant package names, starts lowercase, and omits a trailing period — all spec-friendly. A scope like `scripts` or `build` could have helped filtering, but the subject is short enough that it hardly matters. No body is present, and while the replacement rationale (fewer deps, faster install) is guessable, one sentence would have made the decision auditable later. Since nothing in the shipped code changes, release tooling correctly sees no bump pressure. A tidy maintenance commit whose only cost is the missing one-line rationale.