fix: feat: stack lock protocol for singleton project stack access (#255)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/stack-lock.sh

@@ -98,6 +98,7 @@ stack_lock_acquire() {
   lock_file="$(_stack_lock_path "$project")"
   local deadline
   deadline=$(( $(date -u +%s) + max_wait ))
+  local _rc=0
 
   mkdir -p "$STACK_LOCK_DIR"
 
@@ -130,17 +131,20 @@ stack_lock_acquire() {
         remaining=$(( deadline - $(date -u +%s) ))
         if [ "$remaining" -le 0 ]; then
           echo "[stack-lock] timed out waiting for lock on ${project} (held by ${cur_holder})" >&2
-          return 1
+          _rc=1
+          break
         fi
         echo "[stack-lock] ${project} locked by ${cur_holder}, waiting ${STACK_LOCK_POLL_INTERVAL}s (${remaining}s left)..." >&2
         sleep "$STACK_LOCK_POLL_INTERVAL"
         ;;
       *)
         echo "[stack-lock] unexpected status '${status}' for ${project}" >&2
-        return 1
+        _rc=1
+        break
         ;;
     esac
   done
+  return "$_rc"
 }
 
 # stack_lock_heartbeat <holder_id> <project>