feat: package Arthas skills for agent installers

[zhb0905]

Summary

• standardize the four existing skills under plugins/arthas/skills/<skill-name>/SKILL.md so nested-skill installers discover the same canonical files
• add a validated Codex plugin manifest and repository-scoped marketplace entry
• document Codex Plugin, skills CLI, and GitHub gh-skill installation and update flows in English and Chinese

Why

The previous root-level skills/SKILL.md layout was discovered by some installers but not by tools that follow the skills/*/SKILL.md convention. This change keeps one source of truth while making all four workflows installable through the common agent skill and plugin paths.

Validation

• validated all four skill directories with quick_validate.py
• validated plugins/arthas with validate_plugin.py
• confirmed npx skills add . --list discovers all four skills
• passed gh skill publish --dry-run
• installed arthas-cpu-high from the local repository into a temporary directory with gh skill install --from-local
• previewed all four skills from the pushed fork branch with gh skill preview
• passed JSON parsing, marketplace path checks, and git diff --check

No Java runtime code was changed, so the Maven test suite was not run.

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