Add GitHub App token generation to PR assistant

Author: Kuchizu

.github/workflows/pr-assistant.yml

@@ -13,7 +13,15 @@ jobs:
   pr-assistant:
     runs-on: ubuntu-latest
     steps:
+      - name: Generate GitHub App token
+        id: app-token
+        uses: actions/create-github-app-token@v2
+        with:
+          app-id: ${{ secrets.CODEX_ASSISTANT_APP_ID }}
+          private-key: ${{ secrets.CODEX_ASSISTANT_PRIVATE_KEY }}
+
       - uses: codex-team/action-pr-assistant@master
         with:
           check: description
           mode: draft
+          token: ${{ steps.app-token.outputs.token }}