Add Codex skill integration

[morty-z]

Summary

• add a bundled codex-skill/ entry point for Codex users
• document Codex installation and usage in both English and Chinese READMEs
• allow the new skill directory in .gitignore

What this adds

• codex-skill/SKILL.md with Codex-specific guidance for build, refine, test, and validate workflows
• codex-skill/agents/openai.yaml so the skill is discoverable in Codex
• codex-skill/scripts/install.sh for local installation into $CODEX_HOME/skills or ~/.codex/skills

Why this approach

CLI-Anything already has agent-specific entry layers for Claude Code and OpenCode. This PR adds a minimal Codex adapter without changing the shared HARNESS methodology or the generated Python harness format.

Validation

• ran bash codex-skill/scripts/install.sh with a temporary CODEX_HOME
• verified the installed files included SKILL.md, agents/openai.yaml, and the installer script
• ran git diff --check

Notes

• no generated harness code was changed
• all content is generic and contains no user-specific configuration or private data

[yuh-yang]

LGTM! Please add "Experimental" "Community" tags to the README parts like:

<summary><h4 id="-codex">⚡ Codex <sup><code>Experimental</code></sup> <sup><code>Community</code></sup></h4></summary>