fix: use app-id instead of app-slug for GitHub App token

Aligns with the pattern in PR #1210 and ci-failure-issue.yml.

Author: notgitika

.github/workflows/sync-preview.yml

@@ -17,11 +17,11 @@ jobs:
     name: Merge main into preview
     runs-on: ubuntu-latest
     steps:
-      - name: Generate App token
+      - name: Generate GitHub App Token
         id: app-token
         uses: actions/create-github-app-token@v1
         with:
-          app-slug: agentcore-devx-automation
+          app-id: ${{ vars.APP_ID }}
           private-key: ${{ secrets.APP_PRIVATE_KEY }}
 
       - name: Checkout preview