feat: Add GitHub issue and PR templates (gt-599nf)

Add .github/ISSUE_TEMPLATE/ with bug_report.md and feature_request.md
templates. Add .github/PULL_REQUEST_TEMPLATE.md for PR submissions.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,19 @@
+## Summary
+<!-- Brief description of changes -->
+
+## Related Issue
+<!-- Link to issue: Fixes #123 or Closes #123 -->
+
+## Changes
+<!-- Bullet list of changes -->
+-
+
+## Testing
+<!-- How did you test these changes? -->
+- [ ] Unit tests pass (`go test ./...`)
+- [ ] Manual testing performed
+
+## Checklist
+- [ ] Code follows project style
+- [ ] Documentation updated (if applicable)
+- [ ] No breaking changes (or documented in summary)