ci: close first-time contributor PRs that lack a linked issue (#7164)

* ci: close first-time contributor PRs that lack a linked issue

* ci: indent FTC close comment so the workflow YAML parses

.github/pull_request_template.md

@@ -0,0 +1,24 @@
+## Related issue
+
+Fixes #
+
+<!--
+First-time contributors must link an existing issue in this repo.
+Use a closing keyword: Fixes #123, Closes #123, or Resolves #123.
+PRs without a linked issue are closed automatically.
+-->
+
+## Summary
+
+<!-- Explain the solution and why. -->
+
+## 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". -->