ci(wechat-notify): extract payload building logic to Python script

- Create new `.github/scripts/build_wechat_payload.py` to handle WeChat payload generation
- Replace inline Python string concatenation with dedicated script for better maintainability
- Add checkout step to access the script during workflow execution
- Simplify workflow by delegating payload construction to external script
- Improve code readability and reusability for future notification enhancements

.github/workflows/release-notify-wechat.yml

@@ -79,6 +79,10 @@ jobs:
           echo "EOF" >> $GITHUB_OUTPUT
 
       # 5️⃣ 企业微信通知（Markdown）
+      - name: Checkout for script
+        if: steps.check.outputs.ok == 'true'
+        uses: actions/checkout@v4
+
       - name: Notify WeChat
         if: steps.check.outputs.ok == 'true'
         env:
@@ -89,22 +93,7 @@ jobs:
           PR_URL: ${{ github.event.pull_request.html_url }}
           AI_SUMMARY: ${{ steps.ai.outputs.summary }}
         run: |
-          python3 -c "
-          import json, os
-          content = (
-              '## 🚀 Release 发布通知\n'
-              '> 📦 **分支**: ' + os.environ['BRANCH'] + '\n'
-              '> 👤 **提交人**: ' + os.environ['AUTHOR'] + '\n'
-              '> 📝 **标题**: ' + os.environ['PR_TITLE'] + '\n\n'
-              '### 🧠 AI变更摘要\n'
-              + os.environ['AI_SUMMARY'] + '\n\n'
-              '---\n'
-              '🔗 [查看PR详情](' + os.environ['PR_URL'] + ')'
-          )
-          payload = {'msgtype': 'markdown', 'markdown': {'content': content}}
-          with open('wechat_payload.json', 'w') as f:
-              json.dump(payload, f, ensure_ascii=False)
-          "
+          python3 .github/scripts/build_wechat_payload.py
 
           curl -s "$WECHAT_WEBHOOK" \
             -H 'Content-Type: application/json' \