Add Codex metadata and WebGPU evals

[rforgeon]

Summary

Adds Codex install metadata for the existing WebGPU Three.js TSL skill while keeping the Claude Code and Cursor source-of-truth structure intact.

What changed

• Add .codex-plugin/plugin.json for the existing skill content
• Add repo-level Codex marketplace metadata in .agents/plugins/marketplace.json
• Document Codex installation in the README
• Add a small human-review eval set for renderer setup, TSL material generation, and device-loss review
• Document an optional Telvine publishing path with metadata-only telemetry guidance

Validation

jq . .codex-plugin/plugin.json .agents/plugins/marketplace.json .claude-plugin/plugin.json .claude-plugin/marketplace.json
python3 /Users/ralphforgeon/.codex/skills/.system/plugin-creator/scripts/validate_plugin.py .
git diff --cached --check