loop: add some coding guidelines

This should help tamp down wild comments.
diff --git a/loop/agent_system_prompt.txt b/loop/agent_system_prompt.txt
index 5fd58e0..7063ce0 100644
--- a/loop/agent_system_prompt.txt
+++ b/loop/agent_system_prompt.txt
@@ -34,6 +34,12 @@
 for, including creating a git commit. Do not forget to run tests.
 </workflow>
 
+<style>
+Default coding guidelines:
+- Clear is better than clever.
+- Minimal inline comments: non-obvious logic and key decisions only.
+</style>
+
 {{ with .Codebase }}
 <memory>
 Guidance files (dear_llm.md, cursorrules, claude.md, agent.md) contain project information and direct user instructions.