docs(desktop): record the workbar stories entry

[Astro-Han]

Doc drift left by #4101.

That PR gave Storybook its own features/workbar/stories entry and moved WorkbarSurface out of testing, and taught workbar-boundary.test.ts to accept all three entries — but the feature README still told readers that stories share testing.

This names the three entries and records why the split exists: stories stays out of the production barrel because workbar-host reaches the surface through lazy(), and out of testing because that entry is loaded by node --test against tsc output, while the surface and its tool panels use extensionless relative specifiers only a bundler resolves. Putting it in testing is what broke CI on #4101's first attempt; the README is where the next person should find that out.

Docs only — no code change.

[jackwener]

I found no P0–P3 issues on exact head 994aa9199c10482494046feb591c79aab5ea5e49.

The README now names the three public entries that the code already has: features/workbar, features/workbar/testing, and features/workbar/stories. That matches stories.ts, the production barrel (no WorkbarSurface), workbar-host's lazy(() => import('./workbar-surface')), the Storybook imports, and workbar-boundary.test.ts. testing still does not export the surface. The surface and its panels still use extensionless relative specifiers, so they do not belong on the node --test / tsc path.

Docs only. I am not merging. Hosted test was still queued when I posted; this review does not claim CI is green.

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简体中文

精确 head 994aa9199c10482494046feb591c79aab5ea5e49 上我没有发现 P0–P3。

README 现在写的三个入口和代码一致：生产 barrel、testing、stories。WorkbarSurface 不在生产入口，由 workbar-host 的 lazy() 加载；Storybook 从 stories 引入；testing 仍不导出它。表面和工具面板仍用无扩展名相对路径，所以不能进 node --test / tsc 那条线。

这是文档修正，我不合入。发这条时 hosted test 还在排队，这次审查不表示 CI 已绿。

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