chore: import upstream snapshot with attribution

.github/PULL_REQUEST_TEMPLATE.md

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+# Pull Request
+
+## Linked issue
+
+<!-- Link the issue this PR addresses, e.g. "Closes #123".
+     If there is no issue, briefly say why (e.g. trivial typo fix). -->
+
+Closes #
+
+## What & why
+
+<!-- What does this PR change, and why is the change needed? -->
+
+## How it was tested
+
+<!-- Paste the exact commands you ran and summarise their results. Typical commands
+     (see CONTRIBUTING.md "Running Tests" and "Linting and Type Checking"): -->
+
+```bash
+uv run pytest tests/ --tb=short -q
+uv run ruff check code_review_graph/
+uv run mypy code_review_graph/ --ignore-missing-imports --no-strict-optional
+```
+
+## Checklist
+
+<!-- Mirrors the requirements in CONTRIBUTING.md ("Making Changes" and "Code Style"). -->
+
+- [ ] Tests added for new functionality
+- [ ] All tests pass: `uv run pytest tests/ --tb=short -q`
+- [ ] Linting passes: `uv run ruff check code_review_graph/`
+- [ ] Type checking passes: `uv run mypy code_review_graph/ --ignore-missing-imports --no-strict-optional`
+- [ ] Lines are at most 100 characters
+- [ ] Docs updated where behavior changed (README, `docs/`, docstrings)