perf: preserve conversation prefixes across agent turns

tests/unit/test_memory_config.py

@@ -134,6 +134,8 @@ def test_xml_scrub_depth_positions_the_text_edge():
     ledger.stage_turn([obs(1)])
     ledger.commit_staged(step_key="s1")
     ledger.render([obs(2)])
+    assert PRO_UI_LIST_MARKER in str(ledger.active_messages[0].content)
+    ledger.compact_active()  # depth settings apply at a compaction boundary
     committed = ledger.active_messages[0]
     assert PRO_UI_LIST_MARKER not in str(committed.content)
     assert any(isinstance(b, dict) and b.get("type") == "image_url" for b in committed.content)