docs: refresh READMEs and auth maintenance guidance

[XXPermanentXX]

Summary

• refresh the English and Chinese READMEs with an Agent-first installation and quick-start flow
• simplify the capability overview, add the Managed Agent showcase, and streamline authentication/configuration guidance
• keep the root and npm package READMEs synchronized, including the China-site availability note
• clarify credential domains, workspace scope, and Managed Agent authentication behavior in the auth maintenance guide

Why

The previous quick start read like a long command catalog and did not clearly show the intended Agent + Skill workflow. The published npm README copies also need to stay byte-for-byte aligned with the repository READMEs.

Validation

• assertReadmeSync()
• git diff --check

Docs-only change; no runtime tests were required.