Add GitHub community files

- CONTRIBUTING.md with development guidelines
- Issue templates for bugs and feature requests
- Pull request template

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.github/PULL_REQUEST_TEMPLATE.md

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+## Description
+
+Brief description of the changes.
+
+## Type of Change
+
+- [ ] Bug fix
+- [ ] New feature
+- [ ] Documentation update
+- [ ] Other (describe):
+
+## Testing
+
+Describe how you tested these changes.
+
+## Checklist
+
+- [ ] Code follows project style guidelines
+- [ ] Changes have been tested locally
+- [ ] Documentation updated (if applicable)