feat: add PR and issue templates for contributor workflow (#296)

Adds structured templates to help contributors submit higher-quality
PRs and issues. PR template includes testing checklist (pytest, sync.sh).
Issue templates use YAML forms for bug reports and feature requests.

Fixes #251

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,20 @@
+## Summary
+
+<!-- What does this PR do? 1-3 sentences. -->
+
+## Changes
+
+<!-- Bullet list of what changed. Reference files if helpful. -->
+
+-
+
+## Testing
+
+<!-- How did you verify this works? -->
+
+- [ ] Ran `uv run python -m pytest -q --tb=short`
+- [ ] Ran `bash scripts/sync.sh` (if scripts/ changed)
+
+## Related Issues
+
+<!-- Link issues: Fixes #123 or Relates to #456 -->