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

Original commit 973f27e95 by Teknium targeted _spawn_background_review in
pre-refactor run_agent.py. The body now lives in
agent/background_review._spawn_background_review — re-applied there.

Co-authored-by: Teknium <127238744+teknium1@users.noreply.github.com>

agent/background_review.py

@@ -363,6 +363,21 @@ def _run_review_in_thread(
             # owns the loop and the agent-loop tools dispatch.
             if _parent_api_mode == "codex_app_server":
                 _parent_api_mode = "codex_responses"
+            # skip_memory=True keeps the review fork from
+            # touching external memory plugins (honcho, mem0,
+            # supermemory, etc.).  Without it, the fork's
+            # __init__ rebuilds its own _memory_manager from
+            # config, scoped to the parent's session_id, and
+            # 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).  Built-in
+            # MEMORY.md / USER.md state is re-bound from the
+            # parent below so memory(action="add") writes from
+            # the review still land on disk; the review just
+            # has zero side effects on external providers.
             review_agent = AIAgent(
                 model=agent.model,
                 max_iterations=16,
@@ -374,6 +389,7 @@ def _run_review_in_thread(
                 api_key=_parent_runtime.get("api_key") or None,
                 credential_pool=getattr(agent, "_credential_pool", None),
                 parent_session_id=agent.session_id,
+                skip_memory=True,
             )
             review_agent._memory_write_origin = "background_review"
             review_agent._memory_write_context = "background_review"