fix: remove hardcoded assistant acks after system message injection

s08/s09/s10/s11/s_full inject background-results and inbox as user
  messages before LLM calls. The paired hardcoded assistant "Noted..."
  responses were unnecessary — consecutive user messages work fine and
  the fake acks added noise with no functional value.

  Affected: agents/s08, s09, s10, s11, s_full + docs (zh/en/ja s08, s09)

docs/zh/s08-background-tasks.md

@@ -81,8 +81,6 @@ def agent_loop(messages: list):
             messages.append({"role": "user",
                 "content": f"<background-results>\n{notif_text}\n"
                            f"</background-results>"})
-            messages.append({"role": "assistant",
-                "content": "Noted background results."})
         response = client.messages.create(...)
 ```