docs: add Astron SkillHub and Astron Agent integration guide

Summary

Document how to use the existing gpt-image-2-style-library skill with the iFLYTEK Astron ecosystem, specifically SkillHub and Astron Agent/AstronClaw.

Why

The repository already ships an OpenSkills-style package with a root SKILL.md, local references, and assets. That package shape matches SkillHub's contract, but the README currently documents only local agent installers and the Claude Code marketplace. Astron users therefore have no clear path from this repository to a governed SkillHub package and an Astron Agent skill.

Relevant Astron projects are listed under the iflytek-astron topic. SkillHub's official CLI supports publishing a local skill directory, and Astron Agent can import a published skill and expose its packaged instructions/resources through read_skill_*.

Proposed documentation

• Add an Astron SkillHub / Astron Agent subsection to the Agent Skill docs.
• Show the official @astron-team/skillhub login and publish commands for agents/skills/gpt-image-2-style-library.
• Explain the final Astron Agent/AstronClaw import step and link to the official SkillHub CLI and Astron Agent FAQ documentation.
• Keep the English, Simplified Chinese, and Japanese README variants aligned.

Acceptance criteria

• Commands use placeholders for the registry URL, namespace, and token; no credentials are committed.
• The integration reuses the existing skill directory without copying or changing the generated style data.
• Existing install paths remain unchanged.

I can submit the documentation PR for this proposal.