Avoid duplicate jq pass in bg-pending short-circuit

hooks/loop-codex-stop-hook.sh previously called
has_pending_background_tasks AND count_pending_background_tasks
back-to-back on the short-circuit path. Both wrap
list_pending_background_task_ids, so every pending-bg stop was
running jq over the transcript twice.

Collapse into a single list_pending_background_task_ids call:
capture the id list once, use its non-emptiness for the
short-circuit decision, and derive the count with `wc -l`. The
public helpers has_pending_background_tasks and
count_pending_background_tasks stay for other callers.

No behaviour change. systemMessage wording and exit codes
identical.

Validation (all six suites green):
  - bash tests/run-all-tests.sh -> 1718 passed, 0 failed
  - bash tests/test-stop-hook-bg-allow.sh -> 39 passed, 0 failed
  - HOME=/nonexistent/readonly ... -> 39 passed, 0 failed
  - TZ=America/Los_Angeles ... -> 39 passed, 0 failed
  - TZ=Asia/Tokyo ... -> 39 passed, 0 failed
  - CLAUDE_PROJECT_DIR=/tmp/outer-unrelated ... -> 39 passed, 0 failed

Version stays at 1.16.0.

Author: SihaoLiu

hooks/loop-codex-stop-hook.sh

@@ -137,8 +137,9 @@ fi
 #
 # This check MUST run before any other gate (phase detection, state parsing,
 # branch / plan / git-clean / summary / max-iter checks, Codex review).
-if has_pending_background_tasks "$HOOK_TRANSCRIPT_PATH" "$LOOP_START_TS"; then
-    PENDING_BG_COUNT=$(count_pending_background_tasks "$HOOK_TRANSCRIPT_PATH" "$LOOP_START_TS")
+PENDING_BG_IDS=$(list_pending_background_task_ids "$HOOK_TRANSCRIPT_PATH" "$LOOP_START_TS" 2>/dev/null) || true
+if [[ -n "$PENDING_BG_IDS" ]]; then
+    PENDING_BG_COUNT=$(printf '%s\n' "$PENDING_BG_IDS" | sed '/^$/d' | wc -l | tr -d ' ')
     # Mark the loop as parked; allows the same session to resume later and
     # makes the cross-session guard above reachable if the user opens a
     # different Claude session in this repo before the bg task completes.