chore: update PR template (#36918)

Author: mdrxy

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,6 +1,8 @@
 Fixes #
 
-<!-- Replace everything above this line with a 1-2 sentence description of your change. Keep the "Fixes #xx" keyword and update the issue number. -->
+---
+
+<!-- Keep the `Fixes #xx` keyword at the very top and update the issue number — this auto-closes the issue on merge. Replace this comment with a 1-2 sentence description of your change. No `# Summary` header; the description is the summary. -->
 
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