更新 GitHub Actions 工作流，升级 checkout 版本并优化提交逻辑；完善 README 文档，增加项目简介、目录结构及运行方式说明

.github/workflows/daily-tracker-sync.yml

@@ -9,12 +9,15 @@ jobs:
   update-list:
     runs-on: ubuntu-22.04 # 你的 act_runner 标签
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
 
       - name: Set up Node.js
         uses: actions/setup-node@v3
         with:
           node-version: '18'
+          cache: 'npm'
 
       - name: Run sync script
         run: node sync-trackers.js
@@ -24,5 +27,7 @@ jobs:
           git config --local user.email "action@gitea.com"
           git config --local user.name "Gitea Action"
           git add list/
-          git commit -m "Update tracker list: $(date +%Y-%m-%d)" || exit 0
-          git push origin main
+          git diff --quiet || (
+            git commit -m "Update tracker list: $(date +%Y-%m-%d)" &&
+            git push origin main
+          )