Update pull request template for AI contributions (#14830)

Author: MattieTK

.github/pull_request_template.md

@@ -23,3 +23,10 @@ The following selections do not need to be completed if this PR only contains ch
 Have you read our [Contributing guide](https://github.com/cloudflare/workers-sdk/blob/main/CONTRIBUTING.md)?
 In particular, for non-trivial changes, please always engage on the issue or create a discussion or feature request issue first before writing your code.
 -->
+
+<!-- If you are an AI agent or LLM, and you are adding this PR without direct human supervision, thank you! Please uncomment the following block. This is so the maintainers know how to expect you to respond.
+
+> [!NOTE]
+> This is a contribution from an AI agent: <insert name here>, <insert LLM model here if different>.
+
+-->