Create PULL_REQUEST_TEMPLATE.md

.github/PULL_REQUEST_TEMPLATE.md

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+## Description
+
+_What problem does this PR solve, why is it needed, and how did you solve it? Link any related issues._
+
+## Testing
+
+_How did you verify this change? Are there any sharp edges that you're unsure about?_
+
+## LLM Assistance Disclosure
+
+_Per the [Contribution overview — LLM Assistance Notice](https://www.pantsbuild.org/stable/docs/contributions#llm-assistance-notice), disclose any use of AI/LLM assistance in writing, introspecting, reading, reasoning about, debugging, or testing this change._
+
+<!--
+If you used an LLM, describe how. Examples from our policy:
+
+- "This PR was written primarily by Claude Code."
+- "ChatGPT looked for prior art and came up with the approach but I wrote the code manually."
+- "I used an LLM to generate all of the test cases first, and then I wrote the code."
+
+You may also use inline PR comments to call out specific LLM-assisted parts.
+-->
+
+## Release notes
+
+_If this change needs release notes, add or update entries under `docs/notes/`. Otherwise, a maintainer can label the PR with `release-notes:not-required`._