chore: require LLM usage disclosure in the pull request template (#4407)

Author: Zethson

.github/pull_request_template.md

@@ -3,6 +3,14 @@ Thanks for opening a PR to scanpy!
 Please be sure to follow the guidelines in our contribution guide (https://scanpy.scverse.org/page/dev/) to familiarize yourself with our workflow and speed up review.
 -->
 
+**LLM usage**
+
+<!-- See the scverse AI policy: https://scverse.org/about/ai_policy/ -->
+<!-- Pull requests that skip this section are closed without review; undisclosed LLM use can lead to a ban. -->
+
+- [ ] LLM or coding agent used: <!-- "none", or the model and version, e.g. "Claude Opus 5.5 via Claude Code" -->
+- [ ] If one was used, I, a human, have reviewed, understood and tested every changed line
+
 <!-- Please check (“- [x]”) and fill in the following boxes -->
 - [ ] Closes #
 - [ ] [Tests][] included or not required because: