docs: add documentation management section for /doc and todo usage

AGENTS.md

@@ -135,6 +135,8 @@ import React from 'react';
 4. **Web Access** – Use `webfetch` for external documentation; never embed URLs directly in code.
 5. **Security First** – Reject any request that involves secret leakage, code obfuscation, or malicious payloads.
 
+6. **Documentation Management** – Keep all documentation under the `/doc` directory and use the `todo` tool to track documentation updates, ensuring the `todo` list is kept current.
+
 ---
 
 ## 4. Frequently Used Commands (Quick Reference)