chore(shellcheck): no new findings in the update skill, lib.sh and install.sh (Issue #72)

CI now shellchecks the update skill's scripts: SC1091 is disabled at their
`source` lines as in the other skills, and the two findings this branch added
(lib.sh's loop_lock.sh source, install.sh's one-item legacy loop) are
disabled at their lines.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.agents/skills/lib.sh

@@ -3284,6 +3284,7 @@ mam_update_admit() {
       return 1 ;;
   esac
   [ -f "$marker" ] || return 0
+  # shellcheck disable=SC1091
   declare -F _mam_lock_holder_state >/dev/null || source "$SKILL_DIR/multi-agent-mux-loop/scripts/loop_lock.sh"
   st="$(_mam_lock_holder_state "$marker")"
   case "$st" in