ci(split): 缺少拆分密钥时跳过插件同步

避免新仓库尚未配置 SPLIT_PRIVATE_KEY 时，v6-dev 与 v8-dev 的 Split Repositories 工作流直接失败。

主要内容：

- 增加拆分密钥预检查步骤。

- 未配置 SPLIT_PRIVATE_KEY 时输出 GitHub Actions warning 并跳过 Setup Private Key 与 Split And Push。

- 配置密钥后自动恢复插件仓库拆分推送流程。

.github/workflows/split.yml

@@ -33,9 +33,19 @@ jobs:
             *) echo "Unsupported split branch: ${GITHUB_REF_NAME}" >&2; exit 1 ;;
           esac
 
-      - name: Setup Private Key
+      - name: Check Split Secret
+        id: split_secret
+        run: |
+          if [ -z "$SSH_PRIVATE_KEY" ]; then
+            echo "::warning title=Skip Split::Repository secret SPLIT_PRIVATE_KEY is not configured, skip plugin repository split."
+            echo "enabled=false" >> "$GITHUB_OUTPUT"
+          else
+            echo "enabled=true" >> "$GITHUB_OUTPUT"
+          fi
+
+      - name: Setup Private Key
+        if: steps.split_secret.outputs.enabled == 'true'
         run: |
-          test -n "$SSH_PRIVATE_KEY"
           mkdir -p ~/.ssh
           echo "$SSH_PRIVATE_KEY" > ~/.ssh/id_rsa
           chmod 600 ~/.ssh/id_rsa
@@ -43,6 +53,7 @@ jobs:
           echo "StrictHostKeyChecking no" >> ~/.ssh/config
 
       - name: Split And Push
+        if: steps.split_secret.outputs.enabled == 'true'
         run: |
           git config pull.rebase true
           git config --global user.name "Anyon"