perf: preserve conversation prefixes across agent turns

artemis/context.py

@@ -267,6 +267,9 @@ class ArtemisContext(BaseModel):
     operator's transcript prompt path (``agent.memory.transcript.enabled``).
     Lives beside ``step_memory``; the State never carries message payloads."""
 
+    prompt_prefix_tracker: Any | None = None
+    """Task-local, hash-only prefix diagnostics shared by bound LLM wrappers."""
+
     async def __aenter__(self) -> ArtemisContext:
         return self