docs(blog): draft managed agents architecture article

[xfgong]

Summary

• add English and Simplified Chinese editions comparing sandbox-local and centralized cloud agent harnesses
• cover lifecycle independence, multi-sandbox orchestration, security, recovery, and CubePlex’s architecture direction
• add mobile-first CubePlex-dark architecture diagrams in localized English and Chinese SVG and @2x PNG variants
• place the base English article under blog/ and the Chinese edition under i18n/zh-Hans/docusaurus-plugin-content-blog/
• fix the localized blog-home intro image path

Publication note

A 5:2 cover is still required before final publication.

Validation

• pnpm check
• verified production output contains both English and zh-Hans article pages
• verified English and Chinese pages return 200 and contain their localized titles
• xmllint --noout static/img/blog/managed-agents-architecture/*.svg
• verified localized diagram PNG dimensions: 1920×3400 and 1920×3800
• visually inspected both English diagrams at original resolution