[codex] Support plugin and clone install paths

[rob163]

Summary

• Add Codex plugin marketplace metadata and a Codex plugin manifest for installing the skill through codex plugin.
• Add Claude Code plugin metadata and a skills/cli-panel packaged copy for plugin-based installs.
• Expand README install instructions to cover skills CLI, Codex plugin, Claude Code plugin, and direct git clone installs.
• Extend validation to check plugin metadata and keep the packaged skill copy aligned with the root skill/template.

Validation

• npm run validate
• git diff --cached --check
• CODEX_HOME=$(mktemp -d) codex plugin marketplace add .
• CODEX_HOME=$(mktemp -d) codex plugin add html-as-cli-panel-skill@html-as-cli-panel-skill