fix(install.sh): guard rm -rf with ${SKILLS_DIR:?} (shellcheck SC2115) (#72)

Defense-in-depth against an empty SKILLS_DIR expanding to a root-level path.
Empty values are already rejected upstream; this makes it impossible by
construction. One-char change, behavior otherwise identical.

Author: moonming

install.sh

@@ -69,7 +69,7 @@ count=0
 for dir in "$SRC"/*/; do
   [ -f "${dir}SKILL.md" ] || continue
   name="$(basename "$dir")"
-  rm -rf "${SKILLS_DIR}/${name}"
+  rm -rf "${SKILLS_DIR:?}/${name}"
   cp -R "$dir" "${SKILLS_DIR}/${name}"
   count=$((count + 1))
 done