The apply_ownership journey gains an attach-ceiling case: a group node below
the attach point applies; the attach point itself and a sibling subtree are
both refused (422, message names the attach point). Design doc §7 + CLAUDE.md
record M2 shipped and re-point 'Next' at M3.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>