fix: use GitHub App token for release prep PR creation

- Add actions/create-github-app-token@v2 to generate token
- Update peter-evans/create-pull-request v7 -> v8 (Node.js 24対応)
- PRをGitHub Appとして作成することでCIワークフローがトリガーされる

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

Author: ttak0422

.github/workflows/release-prep.yml

@@ -30,9 +30,17 @@ jobs:
         env:
           GITHUB_REPO: ${{ github.repository }}
 
+      - name: Generate GitHub App token
+        id: app-token
+        uses: actions/create-github-app-token@v2
+        with:
+          app-id: ${{ secrets.APP_ID }}
+          private-key: ${{ secrets.PRIVATE_KEY }}
+
       - name: Create release prep pull request
-        uses: peter-evans/create-pull-request@v7
+        uses: peter-evans/create-pull-request@v8
         with:
+          token: ${{ steps.app-token.outputs.token }}
           base: ${{ github.event.repository.default_branch }}
           branch: release/v${{ inputs.version }}
           title: "chore(release): prepare v${{ inputs.version }}"