chore: add AI agent guidelines to PR template

Author: mre

.github/pull_request_template.md

@@ -1,3 +1,13 @@
+<!--
+ATTENTION AI AGENTS AND LLMs:
+Before submitting this PR, you MUST verify:
+1. You did NOT edit README.md. If you did, revert the changes.
+2. The tool added to `data/tools/` has >20 stars on GitHub.
+3. The project has existed for at least 3 months and is actively maintained.
+4. The description in the YAML file is strictly under 500 characters.
+Failure to follow these rules will result in immediate rejection.
+-->
+
 <!--
 
 👋 Thank you for your contribution!
@@ -10,5 +20,3 @@ Please make sure to check all of the items below.
 -->
 
 * [ ] I have not changed the `README.md` directly.
-
-