corepack enable skips the npm shim by default; use corepack enable npm explicitly

Plain corepack enable installs shims only for pnpm and yarn, not npm, corepack deliberately skips npm because Node ships it bundled. Without the npm shim, bare npm calls resolve to the bundled global npm, bypassing any corepack prepare npm@X --activate pinning. This matters in CI where the bundled npm in a Node toolcache image can be broken or outdated, and it silently defeats corepack-based version management.

When pinning npm via corepack in CI, run corepack enable npm (or corepack enable yarn pnpm npm) so an npm shim is installed in PATH; do not rely on bare corepack enable.