fix(ci): use -B flag to reset sushi30 branch if it already exists

The default branch is now sushi30, so actions/checkout checks it out
and git checkout -b fails. -B creates or resets the branch.

Verified locally with a simulation script before committing.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.github/workflows/sync-rebase.yml

@@ -40,7 +40,7 @@ jobs:
       - name: Attempt rebase of sushi30 onto main
         id: rebase
         run: |
-          git checkout -b sushi30 origin/sushi30
+          git checkout -B sushi30 origin/sushi30
           if git rebase refs/remotes/origin/main; then
             echo "status=success" >> "$GITHUB_OUTPUT"
           else