docs: tighten managed mode quickstart title and summary

[starhaotian]

Cookbook contribution

Summary

• Content type: Recipe（更新已发布内容的元数据）
• Locale: zh-CN
• Demo source included: no
• Target audience: 首次接入 Qoder Cloud Agents API 的开发者
• User problem and expected outcome: 收紧 managed-mode-quickstart 的标题（28→31 字）与 summary（约 100→65 字），保留 Managed Mode 以便与未来 Forward Mode 系列对仗，去掉步骤罗列、价值前置，slug 与正文不变。

Sources and assets

• Public sources used: 无新增
• Asset owner and license: 无外部资产
• All submitted text and assets are original.

Specialist review

• 不适用，仅元数据文案调整。

Author checklist

• I used exactly one author and no more than five approved tags.
• The article has at least three ## sections and no hand-written table of contents.
• I did not use GitHub Alerts, video, raw HTML, JavaScript, MDX, iframe, remote images, SVG, or interactive content.
• Images are local PNG, JPEG, or WebP files with meaningful alternative text and documented rights.（本文无图片）
• The contribution contains no credentials, customer data, personal information, internal links, or unreleased capabilities.
• If a Demo is included, demos/<slug>/ matches the owner article, and the article contains the exact GitHub Demo link.（无 Demo）
• If a Demo is included, its README documents prerequisites, setup, run, verification, cleanup, and cost/safety considerations.（无 Demo）
• Demo configuration uses placeholders; no dependency cache, build output, binary, archive, nested Git metadata, or symbolic link is included.（无 Demo）
• I disclosed the origin and license of Demo source, dependencies, fixtures, and third-party assets.（无第三方资产）
• I reviewed the generated preview locally or from the required GitHub Actions checks.
• Every commit contains a Signed-off-by trailer.
• I understand that maintainer Merge to main is the publication event.

Maintainer Demo review

• Not applicable, or a Maintainer reviewed the Demo README and source manually.
• Not applicable, or the claimed result, cleanup, billable operations, permissions, and destructive side effects were reviewed.