fix(repo): add pull_request_template scaffolding ## Related contract

.github/pull_request_template.md

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+<!--
+Fill in each section and delete these guidance comments before submitting.
+
+The pr-issue-linkage check requires BOTH a native closing keyword AND a
+non-empty ## Related section. It strips HTML comments before validating, so an
+unedited template does not pass until you replace the placeholders with real
+content.
+-->
+
+Closes #
+
+<!--
+Native GitHub closing keyword for the issue this PR closes on merge: replace the
+bare `#` above with the issue number (e.g. Closes #123). Closes/Fixes/Resolves
+are all honored, case-insensitive. If this PR closes no issue, replace the whole
+line with: No linked issue
+-->
+
+## Summary
+
+<!-- What this PR changes and why, in a sentence or two. -->
+
+## Fix
+
+<!-- The concrete change: what you altered and how it addresses the problem. -->
+
+## Verification
+
+<!-- Concrete evidence the change works: commands run, gates passed, output. -->
+
+## Related
+
+<!--
+Issues, PRs, ADRs, or decision-log entries this PR references but does NOT close.
+Required non-empty even when the closing-keyword line above is `No linked issue`
+— if nothing applies, put `N/A`. Use `Refs #N` to link an issue without closing
+it.
+-->