chore: move CodeRabbit high-level summary to walkthrough

Set reviews.high_level_summary_in_walkthrough so the summary appears at
the top of the walkthrough comment instead of the PR description, keeping
the description clean.

Author: sanchitmehtagit

.coderabbit.yaml

@@ -5,6 +5,7 @@ reviews:
   profile: "assertive"
   request_changes_workflow: true
   high_level_summary: true
+  high_level_summary_in_walkthrough: true
   auto_review:
     enabled: true
     drafts: false