add a responsible AI pull request template

.github/pull_request_template.md

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+## Short Description
+
+A one or two-sentence description of what this PR does.
+
+Fixes #
+
+## Details
+
+Add any more details that may be relevant to the reviewer. How did you approach
+these docs? Are there any parts you struggled with, or that need particularly
+careful review?
+
+## AI Usage
+
+Please disclose how you've used AI in this work (it's cool, we just want to
+know!):
+
+- [ ] Code generation (copilot but not intellisense)
+- [ ] Learning or fact checking
+- [ ] Strategy / design
+- [ ] Optimisation / refactoring
+- [ ] Translation / spellchecking / doc gen
+- [ ] Other
+- [ ] I have not used AI
+
+You can read more details in our
+[Responsible AI Policy](https://www.openfn.org/ai#pull-request-templates)