test(issue-26): no test depends on a third-party MQTT broker

- tests/_local_mqtt.py: in-process MQTT 3.1.1 broker on 127.0.0.1 (what
  the repo uses: protocol level 4, clean session, QoS 0/1, retained).
- conftest: one session broker named by MQTT_BROKER/MQTT_PORT/MQTT_TLS;
  other redirecting/auth MQTT_* dropped and restored afterwards.
- Harnesses of the 22 broker-dependent tests (#18 acceptance1/2/3/5,
  #6 t1_1/2/7/8/14/15/16/20, #19 MQTT loop x2, #33 b3/r3/b7, #30
  s9/s10/r2) use it; the "broker unreachable" skips are gone (a local
  broker that does not deliver is a failure). acceptance1/5 and #19 also
  assert on the subscriber, runner and audit logs the real producers write.
- tests/test_local_mqtt.py: conformance with the real clients and the real
  publish_event -> job_subscriber path (C0), an AST guard against public
  broker literals and MQTT_* deleted without a replacement (C2), and the
  session's broker config (C3).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_issue6_fail_closed.py

@@ -16,6 +16,8 @@ from unittest.mock import MagicMock
 
 import pytest
 
+from _local_mqtt import local_broker_env
+
 REPO_ROOT = Path(__file__).resolve().parent.parent
 SKILLS = REPO_ROOT / ".agents" / "skills"
 WRAPPER = SKILLS / "multi-agent-mux-delegate-job" / "multi-agent-mux-delegate-job"
@@ -25,6 +27,7 @@ PUBLISH_PY = SKILLS / "multi-agent-mux-delegate-job" / "scripts" / "publish_even
 
 def _run_wrapper(args, env=None, cwd=None, timeout=60):
     run_env = dict(os.environ)
+    run_env.update(local_broker_env())  # Issue #26: the session's in-process broker
     run_env["DELEGATE_JOB_PYTHON"] = sys.executable
     run_env["AGENT_PYTHON_BIN"] = sys.executable
     if cwd: