ci: install only the headless Chromium shell

[orangeCatDeveloper]

Summary

The Storybook smoke always launches headless, and Playwright picks its executable
by headedness, so playwright install chromium downloaded the full Chrome build
on every Storybook-selected run and never opened it. --only-shell installs only
the build the smoke uses.

No behavior change — the smoke was already launching chromium-headless-shell.
SMOKE_HEADED=1 stops working on a runner; it was only ever meaningful locally,
where developers install their own browsers.

Verification

Download per Storybook-selected run:

before   184.3 MiB full Chrome + 114.7 MiB headless shell + 2.3 MiB FFmpeg = 301.3 MiB, 18s
after                           114.7 MiB headless shell + 2.3 MiB FFmpeg = 117.0 MiB, 13s

Before from job 102913386594 on main, after from this PR's own run. The install after the change, and the
smoke's exact launch call against it:

$ npx playwright install --only-shell chromium && ls "$PLAYWRIGHT_BROWSERS_PATH"
chromium_headless_shell-1234
ffmpeg-1011

$ node -e "chromium.launch({headless:true})..."
LAUNCH OK: ok

$ node -e "chromium.launch({headless:false})..."
browserType.launch: Executable doesn't exist at .../chromium-1234/...

Ran locally: npm run format:check, node --test scripts/ci-workflow-policy.test.mjs
(43 pass). Editing ci.yml selects every surface, so this PR's own test run
exercises the changed step.

AI use

Select exactly one:

• No generative tool made a substantive contribution
• Generative tooling made a substantive contribution

Tool(s) and scope: Claude Code found the unused download while profiling CI step
timings, wrote the change and ran the verification above. Codex (gpt-5.6-sol) and
a second independent Claude session reviewed the diff. Review below is human.

Checklist

• Tests cover the change and fail without it — no test covers a workflow
  install flag; a wrong flag fails the Storybook smoke outright
• Lint, format, typecheck and the affected suites pass locally

Does this PR entail a change in behavior?

• Yes — described under Summary above
• No

[me2seeks]

PR 5187 Review

结论

APPROVE
--only-shell 改动的消费方核实无误：ci.yml 该安装步骤只服务 storybook smoke（scripts/storybook-visual-smoke.mjs:391，headless: process.env.SMOKE_HEADED !== '1'，CI runner 上恒为 headless），而 headed 的 scripts/perf/storybook.mjs:26（headless: false）由独立 workflow performance-frontend.yml:40 自己 playwright install chromium，不受本改动影响。Desktop e2e 用 Electron 自带 Chromium（apps/desktop/e2e/playwright.config.ts 不配 browserType），也不依赖系统 chromium 构建。

发现

无

不确定性（不发到 GitHub 的疑虑）

• SMOKE_HEADED=1 在 CI runner 上从此必失败（PR 描述已承认"only ever meaningful locally"）。若有人真在 CI 里想开 headed 验证，需手工装完整 chromium——可接受，文档化于注释。
• 注释中 "The headed consumer (perf/storybook.mjs) self-installs" 措辞上 perf 脚本本身不含 install 命令，是 perf workflow 自己装；不影响正确性。

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✅ 对抗性审查（第一性原理 + 奥卡姆剃刀 + 证据核对）通过：未发现真实的 P0–P3 问题。@me2seeks 请把关是否 merge。