[ci] Enable CodeRabbit incremental reviews

Add a minimal .coderabbit.yaml that ensures CodeRabbit re-reviews each
push to a PR (incremental review on new commits) and skips drafts.
Without this file, the org-level configuration left incremental reviews
disabled, so only the initial PR open triggered a review.

Signed-off-by: Aleksei Sviridkin <f@lex.la>

.coderabbit.yaml

@@ -0,0 +1,5 @@
+reviews:
+  auto_review:
+    enabled: true
+    auto_incremental_review: true
+    drafts: false