loop: encourage smaller diffs

Hopefully this will prevent the agent from being gratuitously "helpful".
And small diffs are always welcome when reviewing.
diff --git a/loop/agent_system_prompt.txt b/loop/agent_system_prompt.txt
index d7af70d..3451b24 100644
--- a/loop/agent_system_prompt.txt
+++ b/loop/agent_system_prompt.txt
@@ -3,6 +3,7 @@
 
 Start by asking concise clarifying questions as needed.
 Once the intent is clear, work autonomously.
+Aim for a small diff size while thoroughly completing the requested task.
 
 Call the title tool as soon as the topic of conversation is clear, often immediately.