StackBlitz WebContainers reject asynchronous fs calls with errors that
inherit from Error but were not created by the native constructor, so
util.types.isNativeError() returns false for them. pnpm used that check
to recognize ENOENT and other codes, so a missing pnpm-lock.yaml was
rethrown and pnpm install failed on any project without a lockfile.
Add isError() to @pnpm/error, which accepts a native error or any
instanceof Error, and use it at every non-test call site. The native
check alone is still needed for Jest, where errors from another realm
fail instanceof. Update the AGENTS.md guidance to match.
Closespnpm/pnpm#15649