docs: add managed mode quickstart recipe

[starhaotian]

Cookbook contribution

Summary

• Content type: Recipe
• 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: 帮助新用户用五个 curl 命令跑通 Managed Mode 最小闭环——创建 Session、发送消息、通过 SSE 事件流接收 Agent 回复，并读取单次模型调用与 Session 累计的 Credit 消耗。文中含核心概念表（Agent / Environment / Session / Event / Credit）与调用时序图。

Sources and assets

• Public sources used: Qoder Cloud Agents Sessions API 文档（创建 Session、发送事件、事件流、Session 数据结构）
• Asset owner and license: 无外部资产
• All submitted text and assets are original.

Specialist review

• 不适用。文中 API 示例已基于真实环境完成端到端验证（创建 → 发消息 → SSE 事件流 → Credit 查询），响应示例均来自实测输出。

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.（示例中的 ID 均为尖括号占位符）
• 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.