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_issue30_stall_watch.py

@@ -488,12 +488,7 @@ def test_scoped_env_drops_the_pane_inherited_workspace_for_name_lookups_only():
 import test_issue18_liveness_contract as t18  # noqa: E402
 
 
-LIVE_BROKER = ("MQTT_BROKER", "MQTT_PORT", "MQTT_TLS", "MQTT_USERNAME", "MQTT_PASSWORD")
-
-
-def _live_broker() -> dict:
-    """This environment's broker (the #18 harness defaults to a public one)."""
-    return {k: os.environ[k] for k in LIVE_BROKER if k in os.environ}
+from _local_mqtt import local_broker_env  # noqa: E402  (Issue #26: the session's in-process broker)
 
 
 def _watchers(jid: str) -> list:
@@ -507,7 +502,7 @@ def test_s9_the_wrapper_runs_the_watcher_for_the_jobs_life(tmp_path, mock_herdr,
     reg.mkdir()
     t18._headless_agent(Path(mock_herdr), "headless-codex")
     proc, env = t18._submit(tmp_path, reg, "herdr:headless-codex", "codex", ["--ack-timeout", "40"],
-                            env_extra={"MAM_STALL_WATCH": switch, **_live_broker()}, timeout_s=60)
+                            env_extra={"MAM_STALL_WATCH": switch, **local_broker_env()}, timeout_s=60)
     jid = t18._wait_job_id(reg)
     try:
         time.sleep(1)
@@ -533,7 +528,7 @@ def test_s10_remote_and_human_jobs_are_not_watched(tmp_path, agent, session, why
     reg = tmp_path / "jobs"
     reg.mkdir()
     proc, env = t18._submit(tmp_path, reg, session, agent, ["--ack-timeout", "40"], timeout_s=60,
-                            env_extra={"MAM_DISPATCH_SECRET": "s10-secret", **_live_broker()})
+                            env_extra={"MAM_DISPATCH_SECRET": "s10-secret", **local_broker_env()})
     jid = t18._wait_job_id(reg)
     try:
         time.sleep(1)
@@ -620,7 +615,7 @@ def test_r2_the_loop_relays_a_blocked_notice_while_the_job_is_open(mam_sandbox,
                                "SKS_IDLE_TIMEOUT_MS": "500", "SKS_EMPTY_GIVEUP": "1",
                                "MAM_ACK_TIMEOUT_S": "40", "MAM_JOBS_DIR": str(reg),
                                "PYTHONPATH": str(SKILLS), "PYTHONUNBUFFERED": "1"})
-    env.update(_live_broker())
+    env.update(local_broker_env())
     script = '''from lib_py.loop.runner import LoopRunner
 r = LoopRunner(["--creator", "headless-codex", "--task", "review check", "--job-timeout", "60"])
 r.parse_args()