fix(run_agent): isolate background review fork from external memory plugins (#27190)

Pass skip_memory=True to the AIAgent constructor used by
_spawn_background_review() so the review fork's __init__ no longer
rebuilds a _memory_manager wired to honcho / mem0 / supermemory /
etc. under the parent's session_id.

Before this change, the review fork ingested its harness prompt
(the 'Review the conversation above and update the skill library...'
text) into the user's real memory namespace via three sites in
run_conversation():
  - on_turn_start(turn_count, prompt)      cadence + turn-message
  - prefetch_all(prompt)                   recall query
  - sync_all(prompt, review_output, ...)   harness + review output
                                           recorded as a
                                           (user, assistant) pair

Built-in MEMORY.md / USER.md state is still rebound from the parent
right after construction, so memory(action='add') writes from the
review continue to land on disk; only the external-plugin side
effects are removed.

Reported by @Utku.

tests/run_agent/test_background_review.py

@@ -193,3 +193,51 @@ def test_background_review_summary_is_attributed_to_self_improvement_loop(monkey
     assert captured_bg_callback[0].startswith("💾 Self-improvement review:"), (
         captured_bg_callback[0]
     )
+
+
+def test_background_review_fork_skips_external_memory_plugins(monkeypatch):
+    """The background review fork must NOT touch external memory plugins.
+
+    Without skip_memory=True on the fork constructor, AIAgent.__init__
+    rebuilds its own _memory_manager from config, scoped to the parent's
+    session_id.  The review fork's run_conversation() then leaks the
+    harness prompt into the user's real memory namespace via three
+    ingestion sites: on_turn_start (cadence + turn message),
+    prefetch_all (recall query), and sync_all (harness prompt + review
+    output recorded as a (user, assistant) turn pair).  The fix is a
+    single kwarg on the fork constructor — this test guards it.
+    """
+    captured_kwargs: dict = {}
+
+    class FakeReviewAgent:
+        def __init__(self, **kwargs):
+            captured_kwargs.update(kwargs)
+            self._session_messages = []
+
+        def run_conversation(self, **kwargs):
+            pass
+
+        def shutdown_memory_provider(self):
+            pass
+
+        def close(self):
+            pass
+
+    monkeypatch.setattr(run_agent_module, "AIAgent", FakeReviewAgent)
+    monkeypatch.setattr(run_agent_module.threading, "Thread", ImmediateThread)
+
+    agent = _bare_agent()
+
+    AIAgent._spawn_background_review(
+        agent,
+        messages_snapshot=[{"role": "user", "content": "hello"}],
+        review_memory=True,
+    )
+
+    assert captured_kwargs.get("skip_memory") is True, (
+        "Background review fork must be constructed with skip_memory=True "
+        "so AIAgent.__init__ does not rebuild a _memory_manager wired to "
+        "external plugins (honcho, mem0, supermemory, ...).  Without this "
+        "the fork leaks harness prompts into the user's real memory "
+        "namespace via on_turn_start / prefetch_all / sync_all."
+    )