Add org-default PR template

.github/PULL_REQUEST_TEMPLATE.md

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+<!-- Size and risk labels are auto-applied by CI. -->
+
+## Summary
+<!--
+What changed and why. For bug fixes: problem, impact, evidence, fix.
+For features: motivation and approach.
+Reviewers should be able to understand the change without reading the diff first.
+-->
+
+## Review guidance
+- **Urgent** (needs same-day review): no
+- **High complexity** (non-obvious logic, careful review): no
+- **Estimated review time**: <!-- 15 min / 30 min / 1 hr+ -->
+- **Key files to focus on**: <!-- e.g., path/to/file.go -->
+- **Areas you're unsure about**: <!-- Where you'd appreciate extra scrutiny -->
+
+<!-- For large or complex PRs (XL size, high complexity, or spanning multiple -->
+<!-- teams/domains): schedule a live review session with the relevant reviewers -->
+<!-- rather than waiting for async review. This prevents big PRs from sitting -->
+<!-- in the queue while reviewers put them off. -->
+
+## Testing
+<!--
+What did you test, or what should be tested before merge?
+Use prose, a `- [ ]` checklist of real verification steps (with `[x]` for
+items you've already done), or both. Only use `- [ ]` for actual tasks --
+not for selectors or "if applicable" toggles -- so the PR's task counter
+stays accurate.
+-->
+
+## Deployment notes
+<!-- Anything the on-call or deployer should know? Migrations, feature flags, rollback plan? -->