fix(issue-19): arm the subscriber's stop handlers before it connects; read $! with nounset off

Review round 3 (reviewer-grok-01, NOT PASS N1; non-blocking B1):

- reconcile.sh (N1): the python subscriber installs its SIGTERM/SIGINT
  handlers before connect/loop_start. A signal during the broker handshake,
  the CONNACK wait, or right after the paho thread logs "subscribed to" now
  takes the clean path (the signal line, loop_stop/disconnect, rc 0) instead
  of Python's default action; the CONNACK wait ends on it, and a signalled
  subscriber never falls back to polling (rc 3).
- reconcile.sh (B1): `_sub_stop` reads `$!` with nounset off. On bash 3.2
  `${!:-}` and `${!+x}` are both unbound before the first `&`, so the trap
  died with rc 1 instead of 143.
- tests: SIGTERM during a CONNACK wait against a silent local listener (no
  broker needed), and the verbatim `_sub_stop` run without and with a
  child; both fail on 8f9745b.
- docs: monitor SKILL.md and VERSIONS.md state the handler order; test
  count 16.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HCGpJXhSZasJLU38ksDghe

.agents/skills/multi-agent-mux-monitor/scripts/reconcile.sh

@@ -80,8 +80,15 @@ if [ "$SUBSCRIBE" = "1" ]; then
   _sub_stop() {
     trap - TERM INT
     # `$!` covers the window between `&` and `_sub_pid=$!` (review R-1); a
-    # child that already exited makes kill/wait harmless no-ops.
-    local child="${_sub_pid:-${!:-}}"
+    # child that already exited makes kill/wait harmless no-ops. Before the
+    # first `&` bash 3.2 treats `$!` as unbound under `set -u` - `${!:-}` and
+    # `${!+x}` included - so it is read with nounset off (review B1).
+    local child="${_sub_pid:-}"
+    if [ -z "$child" ]; then
+      set +u
+      child="$!"
+      set -u
+    fi
     if [ -n "$child" ]; then
       kill -TERM "$child" 2>/dev/null || true
       wait "$child" 2>/dev/null || true
@@ -290,28 +297,6 @@ def on_connect(_c, _u, _flags, reason_code, _props):
         print(f"MQTT Monitor connection failed: rc={rc}", flush=True)
 
 
-cfg = mqtt_common.broker_config_from_env()
-client = mqtt_common.make_client("monitor_sub", cfg)
-client.on_message = on_message
-client.on_connect = on_connect
-print(f"MQTT Monitor: connecting to {cfg.host}:{cfg.port} (TLS={cfg.tls})...", flush=True)
-
-# Connection failure → fall back to polling (review item 4).
-try:
-    client.connect(cfg.host, cfg.port, cfg.keepalive)
-except Exception as e:
-    print(f"MQTT Monitor: connect failed ({e}); falling back to polling", flush=True)
-    sys.exit(3)
-
-client.loop_start()
-_wait = time.time()
-while time.time() - _wait < 5 and not state['connected'] and not state['failed']:
-    time.sleep(0.1)
-if not state['connected']:
-    print("MQTT Monitor: broker did not accept connection; falling back to polling", flush=True)
-    client.loop_stop()
-    sys.exit(3)
-
 import threading, signal
 stop_event = threading.Event()
 state['stop_signal'] = None
@@ -322,8 +307,39 @@ def _on_stop_signal(signum, _frame):
     stop_event.set()
 
 
+# Issue #19 (review N1): the handlers go in before connect/loop_start. The paho
+# thread logs "subscribed to" from on_connect, and a SIGTERM during the broker
+# handshake or right after that line must take the same clean path as a later
+# one - the "signal N received" line and loop_stop/disconnect - not Python's
+# default action. A signalled subscriber exits 0, never 3: rc 3 would send the
+# bash wrapper into the polling fallback.
 signal.signal(signal.SIGTERM, _on_stop_signal)
 signal.signal(signal.SIGINT, _on_stop_signal)
+
+cfg = mqtt_common.broker_config_from_env()
+client = mqtt_common.make_client("monitor_sub", cfg)
+client.on_message = on_message
+client.on_connect = on_connect
+print(f"MQTT Monitor: connecting to {cfg.host}:{cfg.port} (TLS={cfg.tls})...", flush=True)
+
+# Connection failure → fall back to polling (review item 4).
+try:
+    client.connect(cfg.host, cfg.port, cfg.keepalive)
+except Exception as e:
+    if state['stop_signal'] is None:
+        print(f"MQTT Monitor: connect failed ({e}); falling back to polling", flush=True)
+        sys.exit(3)
+else:
+    client.loop_start()
+    _wait = time.time()
+    while (time.time() - _wait < 5 and not state['connected'] and not state['failed']
+           and not stop_event.is_set()):
+        stop_event.wait(0.1)
+    if not state['connected'] and state['stop_signal'] is None:
+        print("MQTT Monitor: broker did not accept connection; falling back to polling", flush=True)
+        client.loop_stop()
+        sys.exit(3)
+
 start = time.time()
 try:
     while True: