`sibling` asks for a git URL, clones the repo into
~/.local/share/siblings/<name>, runs its install.sh and confirms the
install. `sibling list` and `sibling remove` manage installed siblings.
updates.d/71-siblings.sh pulls every sibling during `update` and runs
its update.sh, reporting failures in the update summary.
Document the sibling contract in skills/sibling-project-format.md and
update the custom-hook guide to match.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
