docs: add community standard files (#257)

.github/pull_request_template.md

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+## Summary
+
+- what changed
+- why it changed
+
+## Impact
+
+- user-facing impact:
+- developer/maintainer impact:
+
+## Testing
+
+- [ ] `bun run build`
+- [ ] `bun run smoke`
+- [ ] focused tests:
+
+## Notes
+
+- provider/model path tested:
+- screenshots attached (if UI changed):
+- follow-up work or known limitations: