Sync scripts/signals.sh with docker-common's canonical version

Adopts bash-arithmetic exit-code computation ($((128 + SIGNUM))) instead
of piping to bc, dropping that dependency. Consolidates this project
onto docker-common's shared scripts (see that repo's CLAUDE.md).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NiNnj78HGx1KWyCCo39HSz

scripts/signals.sh

@@ -21,5 +21,5 @@ signal_name_to_number() {
 signal_return_val() {
     SIGNAME="$1"
     SIGNUM=$(signal_name_to_number ${SIGNAME})
-    echo "128 + $SIGNUM" | bc
+    echo $((128 + SIGNUM))
 }