chore(creator-studio): stop tracking the build output

assets/creator-studio/build is generated by `npm run build`, so committing it
put a rebuilt bundle in every diff. It now follows the same deploy-only rule
as assets/admin/build.

- Root .gitignore ignores build/* and node_modules/, keeping a .gitkeep so
  the directory still exists in a clean checkout.
- Drops the three tracked artifacts from the index.
- The contract test no longer requires the artifacts to exist; it checks the
  set is all-or-nothing, so a clean checkout and a locally built tree both
  pass. This mirrors admin-build-graph-contract.
- README documents that build/ is produced on the target machine.

887 widget-system tests pass.

assets/creator-studio/README.md

@@ -10,6 +10,8 @@ npm install            # 首次安装（packages 已锁定 WordPress 6.4 时代
 npm run build          # 产物写入 assets/creator-studio/build/
 ```
 
+`build/` 是构建产物，不进版本库（与 `assets/admin/build` 同一约定）；部署时在目标机上执行上面的 `npm run build`。
+
 PHP 读取 `build/index.asset.php`（依赖清单 + 版本）并只在投稿模板加载，构建产物缺失时自动回退到原有 TinyMCE 投稿表单。
 
 ## 使用前提