docs: add pull request governance checklist

Author: LalaSkye

.github/pull_request_template.md

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+## Change class
+
+- [ ] code primitive
+- [ ] governance claim
+- [ ] invariant
+- [ ] adversarial vector
+- [ ] receipt / audit surface
+- [ ] documentation only
+
+## Boundary check
+
+- [ ] This change does not widen public claims without evidence.
+- [ ] This change does not extend `EXIT_ENUM`.
+- [ ] This change does not weaken scope monotonicity.
+- [ ] This change does not allow unresolved state to bind into execution.
+
+## Proof surface
+
+- [ ] Tests or verifier command updated where needed.
+- [ ] Build receipt updated where needed.
+- [ ] CI green proof is not claimed unless observed.