[codex] Fix plugin packaging layout

[rob163]

Summary

• Move the Codex and Claude plugin manifests into a self-contained plugin package at plugins/html-as-cli-panel-skill.
• Make the root SKILL.md, root assets/cli-panel.html, and skills/cli-panel entries symlink into that package instead of duplicating the template.
• Point Codex and Claude marketplace entries at the packaged plugin directory.
• Update validation to check symlink entrypoints, plugin metadata, and installed skill/template paths.

Why

PR #3 made the install commands appear to succeed, but the Codex plugin cache only received the codex subdirectory. The codex/skills -> ../skills symlink pointed outside the cached plugin root, so the actual skill package was not installed. This layout keeps the plugin self-contained while preserving direct git clone and skills CLI entrypoints.

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
• Verified the installed Codex plugin cache contains skills/cli-panel/SKILL.md and skills/cli-panel/assets/cli-panel.html.