The -Pack failure path told an outside reader to expect "the shared workspace
feed beside the estates" — vocabulary describing a private multi-repo checkout,
in the one file a contributor runs first. It now says what the folder is for and
what to do instead, which is the same information without the map.

Found by re-sweeping the tree after the flip rather than before it: the sweep
that mattered was the one run against what is actually public.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>