perf: preserve conversation prefixes across agent turns

tests/unit/agents/test_operator_transcript.py

@@ -355,7 +355,7 @@ async def test_transcript_mode_two_turns_build_four_regions(tmp_path):
         # Text edge at depth 1: the committed observation has lost its plan
         # recitation as soon as the next tail exists (its screenshot stays
         # until the screenshot edge).
-        assert PLAN_RECITATION_MARKER not in str(committed_obs.content)
+        assert PLAN_RECITATION_MARKER in str(committed_obs.content)  # retained until compaction
         # The live tail still recites the plan.
         assert PLAN_RECITATION_MARKER in str(turn2_messages[-1].content)
         # Message layout is S + A + tail.