Function RunPackageInstall

Runs package install from the monorepo root using the appropriate package manager — or, when the repo is a member of an mj dev workspace, from that workspace's parent with pnpm (see ResolveInstallRoot); the result says where it ran.

Registry configuration is resolved in this order:

1. Explicit registryUrl parameter (passed from manifest)
2. Scoped registry from .npmrc / .pnpmrc (automatic — package managers read these natively)
3. Default registry for the package manager