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_issue33_budget.py

@@ -25,6 +25,8 @@ from pathlib import Path
 
 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"
@@ -48,10 +50,12 @@ def _env(home: Path, extra=None, broker: bool = False) -> dict:
     for k in ("MAM_JOBS_DIR", "MAM_REAL_ROOT", "MAM_JOB_ID", "MAM_JOB_TIMEOUT_S", "MAM_CHILD_WAIT_S",
               "MAM_WAIT_GRACE_S"):
         env.pop(k, None)
-    if not broker:
+    for k in ("MQTT_BROKER", "MQTT_HOST", "MQTT_PORT", "MQTT_TLS", "MQTT_USERNAME", "MQTT_PASSWORD"):
+        env.pop(k, None)
+    if broker:
+        env.update(local_broker_env())  # Issue #26: the session's in-process broker
+    else:
         env["MAM_ENV_FILE"] = str(home / ".mam.env.absent")
-        for k in ("MQTT_BROKER", "MQTT_HOST", "MQTT_PORT", "MQTT_TLS", "MQTT_USERNAME", "MQTT_PASSWORD"):
-            env.pop(k, None)
         env.update(UNREACHABLE)
     if extra:
         env.update({k: str(v) for k, v in extra.items()})