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>

updates.d/71-siblings.sh

@@ -0,0 +1,27 @@
+#!/bin/bash
+
+# Sibling projects installed with the `sibling` command
+SIBLINGS="$HOME/.local/share/siblings"
+
+is_installed() {
+  compgen -G "$SIBLINGS/*/.git" > /dev/null
+}
+
+run_update() {
+  echo "Updating sibling projects..."
+  local dir name failed=0
+  for dir in "$SIBLINGS"/*/; do
+    [ -d "$dir/.git" ] || continue
+    name="$(basename "$dir")"
+    echo "  $name"
+    if ! git -C "$dir" pull --ff-only --quiet; then
+      echo "  Could not pull $name."
+      failed=1
+      continue
+    fi
+    if [ -f "$dir/update.sh" ]; then
+      (cd "$dir" && bash update.sh) || { echo "  $name's update.sh failed."; failed=1; }
+    fi
+  done
+  return $failed
+}