Add sibling command for installing and updating sibling projects

`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>

update

@@ -41,6 +41,7 @@ fi
 mkdir -p "$COREDESKTOP/updates.d"
 install_script "$COREDESKTOP/update" ~/.local/bin/update
 install_script "$COREDESKTOP/refresh" ~/.local/bin/refresh
+install_script "$COREDESKTOP/sibling" ~/.local/bin/sibling
 
 if [[ -z "$UPDATE_REEXECED" ]]; then
     export UPDATE_REEXECED=1