docs: add pull request template

.github/pull_request_template.md

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+## Description
+
+<!-- Provide a concise summary of your changes and the motivation behind them. -->
+
+## Related Issue
+
+<!-- Link the issue this PR addresses. Use "Closes #<number>" to auto-close it on merge. -->
+
+Closes #
+
+## Type of Change
+
+<!-- Check all that apply. -->
+
+- [ ] Bug fix (non-breaking change that fixes an issue)
+- [ ] New feature (non-breaking change that adds functionality)
+- [ ] Breaking change (fix or feature that would cause existing functionality to change)
+- [ ] Documentation update
+- [ ] Refactoring / code cleanup (no functional changes)
+- [ ] Test update (adding or modifying tests only)
+
+## Component(s) Affected
+
+<!-- Check all that apply. -->
+
+- [ ] Library (`library/`)
+- [ ] Prototype / Streamlit app (`prototype/`)
+- [ ] Data pipeline (`data/`)
+- [ ] Analysis / Notebooks (`analysis/`)
+- [ ] Documentation (`docs/`)
+- [ ] CI / GitHub Actions (`.github/`)
+- [ ] Other (please specify):
+
+## How Has This Been Tested?
+
+<!-- Describe how you tested your changes (e.g., `uv run pytest`, manual testing). -->
+
+## Checklist
+
+- [ ] I have read the [CONTRIBUTING.md](../CONTRIBUTING.md) guide.
+- [ ] My changes follow the existing code style (checked with `uv run ruff check`).
+- [ ] I have added or updated tests as appropriate.
+- [ ] All tests pass locally (`uv run pytest`).
+- [ ] I have updated documentation if needed.