fix(slice-1): dev.sh portable on bash 3.2 — drop wait -n, reap process subtree

macOS ships bash 3.2 (no wait -n): poll with kill -0 instead. npm spawns vite as a
grandchild, so cleanup now recurses pgrep -P to reap the whole subtree — Ctrl-C frees
both :8000 and :5173 cleanly. Verified clean-checkout -> dev.sh -> empty self-migrated
Postgres -> /healthz 200, and trap teardown.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

scripts/dev.sh

@@ -35,13 +35,30 @@ backend_pid=$!
 ( cd "$repo_root/frontend" && npm run dev ) &
 frontend_pid=$!
 
+# Kill a process and its whole descendant tree (children first), portably across
+# macOS and Linux — npm spawns vite as a grandchild, so killing the captured pid
+# alone would orphan it on :5173. `pgrep -P` (list children) exists on both.
+kill_tree() {
+  local pid=$1 child
+  for child in $(pgrep -P "$pid" 2>/dev/null); do
+    kill_tree "$child"
+  done
+  kill "$pid" 2>/dev/null || true
+}
+
 cleanup() {
+  trap - INT TERM
   echo
   echo "==> stopping backend and Vite"
-  kill "$backend_pid" "$frontend_pid" 2>/dev/null || true
+  kill_tree "$backend_pid"
+  kill_tree "$frontend_pid"
   wait "$backend_pid" "$frontend_pid" 2>/dev/null || true
 }
 trap cleanup INT TERM
 
-wait -n "$backend_pid" "$frontend_pid"
+# Wait until either process exits, then tear the other down. `wait -n` would be
+# tidier but is unsupported on macOS's bundled bash 3.2, so poll portably.
+while kill -0 "$backend_pid" 2>/dev/null && kill -0 "$frontend_pid" 2>/dev/null; do
+  sleep 1
+done
 cleanup