docs: define issue-first contribution workflow

.github/PULL_REQUEST_TEMPLATE.md

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+## Ready issue
+
+Implements #<issue-number>
+
+<!--
+Replace the placeholder above with exactly one GitHub issue that has the
+`state:ready` label. Use "Implements", not "Closes", "Fixes", or "Resolves":
+the issue remains open for human verification after this PR is merged.
+-->
+
+## Summary
+
+<!-- Explain the solution and how it follows the design agreed in the issue. -->
+
+## Verification
+
+<!-- List the automated and manual checks used to verify the change. -->
+
+- [ ] Tests added or updated for the changed behavior
+- [ ] Relevant tests and quality checks pass locally
+
+## Additional context
+
+<!-- Include screenshots, compatibility notes, follow-up work, or "None". -->