Add PR template

.github/PULL_REQUEST_TEMPLATE.md

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+### Purpose
+<!-- What problem does this PR solve, or what capability does it add? Provide a link to any relevant issues or pull requests. -->
+
+### Breaking changes and changes to results
+<!-- Does this PR require changes to existing input files, scripts, or workflows?
+     Does it change the BOUT++ API or ABI, i.e. will it need require downstream
+     changes in BOUT++ applications or
+     Does it change physics results for existing inputs?
+     Describe the changes, their rationale and any migration steps needed.
+     Are there any API or ABI breaking changes? -->
+
+### Change Summary
+<!-- Briefly describe what was changed. -->
+
+### Validation
+<!-- How did you verify this works? Include relevant tests, manual checks, or analysis. If not tested, say why. -->
+
+### AI Assistance
+<!-- Were AI tools used during development? If yes, briefly describe how, and how you verified their output. Remove this section if your PR does not contain AI-generated content. -->
+
+### Documentation
+<!-- What documentation was updated? If none, explain why no documentation changes were needed. -->
+
+### Review Notes
+<!-- Anything reviewers should pay particular attention to? For example: risks, limitations, things you're not sure about. -->