test(review): add integration guards for the two isolation wirings (review)

Phase 2c mutation-check found the salvaged tests covered only the pure helpers
(_is_background_review_harness_message / _strip_background_review_harness) — the
two integration WIRINGS had zero coverage: removing the _persist_disabled guard
in _flush_messages_to_session_db, or the _strip call in
get_messages_as_conversation, left all 13 tests green.

Add:
- TestPersistDisabledHardStop: a _persist_disabled agent's flush writes nothing
  to a live SessionDB (guards the run_agent hard-stop).
- TestGetMessagesAsConversationStripsHarness: a session with stray harness rows
  resumes clean end-to-end through get_messages_as_conversation (guards the
  hermes_state load-time wiring).
Mutation-checked: each new test fails when its wiring is reverted.

tests/test_background_review_session_isolation.py

@@ -102,3 +102,83 @@ class TestStripBackgroundReviewHarness:
         ]
         out = _strip_background_review_harness(messages)
         assert [m["content"] for m in out] == ["real question", "real answer"]
+
+
+class TestGetMessagesAsConversationStripsHarness:
+    """The load-on-read wiring: get_messages_as_conversation must actually call
+    _strip_background_review_harness, so a session polluted with stray harness
+    rows resumes clean end-to-end (not just the pure helper in isolation)."""
+
+    def test_polluted_session_resumes_without_harness(self):
+        import tempfile
+        from pathlib import Path
+        from hermes_state import SessionDB
+
+        with tempfile.TemporaryDirectory() as tmp:
+            db = SessionDB(db_path=Path(tmp) / "t.db")
+            try:
+                db.create_session(session_id="s1", source="cli")
+                db.append_message("s1", role="user", content="What's the weather?")
+                db.append_message("s1", role="assistant", content="It's sunny.")
+                # Stray background-review pollution written by an older build.
+                db.append_message(
+                    "s1", role="user",
+                    content="Review the conversation above and update the skill library with anything useful.",
+                )
+                db.append_message("s1", role="assistant", content="I'll act as the curator now.")
+                db.append_message("s1", role="user", content="Thanks, now book a flight.")
+
+                conv = db.get_messages_as_conversation("s1")
+                contents = [m["content"] for m in conv]
+
+                # Harness user turn AND its curator-mode assistant reply are gone.
+                assert not any(
+                    isinstance(c, str) and c.lstrip().startswith("Review the conversation above")
+                    for c in contents
+                )
+                assert "I'll act as the curator now." not in contents
+                # Genuine turns survive in order.
+                assert contents == ["What's the weather?", "It's sunny.", "Thanks, now book a flight."]
+            finally:
+                db.close()
+
+
+class TestPersistDisabledHardStop:
+    """The isolation wiring: a _persist_disabled agent must never write to the
+    session store via _flush_messages_to_session_db, even with a live db set."""
+
+    def test_flush_is_a_noop_when_persist_disabled(self):
+        import os
+        import tempfile
+        from pathlib import Path
+        from unittest.mock import patch
+        from hermes_state import SessionDB
+
+        with tempfile.TemporaryDirectory() as tmp:
+            db = SessionDB(db_path=Path(tmp) / "t.db")
+            try:
+                with patch.dict(os.environ, {"OPENROUTER_API_KEY": "test-key"}):
+                    from run_agent import AIAgent
+                    agent = AIAgent(
+                        api_key="test-key",
+                        base_url="https://openrouter.ai/api/v1",
+                        model="test/model",
+                        quiet_mode=True,
+                        session_db=db,
+                        session_id="s-review",
+                        skip_context_files=True,
+                        skip_memory=True,
+                    )
+                agent._ensure_db_session()
+                agent._persist_disabled = True
+
+                agent._flush_messages_to_session_db(
+                    [{"role": "user", "content": "Review the conversation above and update the skill library."},
+                     {"role": "assistant", "content": "curator reply"}],
+                    [],
+                )
+
+                # Nothing written: the hard-stop fired before any append.
+                assert db.get_messages("s-review") == []
+            finally:
+                db.close()