chore: suppress verbose review body in Copilot PR reviews (#39761)

Author: yury-s

.github/copilot-instructions.md

@@ -7,3 +7,4 @@ When reviewing pull requests:
 - Keep each comment short — one or two sentences maximum.
 - Do not write long descriptions or summaries of what the code does.
 - Do not suggest refactors or improvements unrelated to the PR's stated goal.
+- Do not produce a review body or top-level summary. Leave the review body empty; put findings only in inline comments.