docs(repo): add contributing guide and pr template

.github/pull_request_template.md

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+## Summary
+
+- What changed?
+- Why was it needed?
+
+## Scope
+
+- In scope:
+- Out of scope:
+
+## Validation
+
+- [ ] `go test ./... -count=1`
+- [ ] `go test -race ./...`
+- [ ] `go vet ./...`
+- [ ] If relevant: `bash -n scripts/verify_preprod_gate_b.sh`
+- [ ] If relevant: `bash -n scripts/verify_gate_c_rollback.sh`
+
+## Risk
+
+- What could regress?
+- What areas need reviewer attention?
+
+## Config / Docs Impact
+
+- [ ] No config contract change
+- [ ] Updated `docs/CONFIG_CONTRACT_BASELINE.md`
+- [ ] Updated runbook / QA / plan docs if behavior changed
+
+## Notes
+
+- Related issues / design docs / rollout notes