feat: add Codex plugin marketplace

[ankitranjan7]

Summary

• Adds .codex-plugin/plugin.json for Webcmd.
• Adds .agents/plugins/marketplace.json so Codex can discover Webcmd as a marketplace plugin.
• Includes the Codex plugin files in npm package metadata.

Reasoning

Codex users should not need to run both npx skills add and a Codex-specific setup. The Codex-native path is a plugin marketplace entry that bundles the Webcmd discovery skill. That matches the pattern used by Codex plugin projects: a plugin manifest at .codex-plugin/plugin.json, marketplace metadata under .agents/plugins/marketplace.json, and install through codex plugin marketplace add ... followed by /plugins.

This PR is stacked on the single-discovery-skill PR because the Codex plugin should expose only skills/webcmd, not the old multi-skill install surface.

Industry practice followed

• Use Codex plugin packaging for Codex-only users instead of asking them to install generic skills manually.
• Keep plugin metadata in the standard .codex-plugin/plugin.json location.
• Keep marketplace metadata in .agents/plugins/marketplace.json, using a Git-backed source entry for this repo.
• Point the plugin at ./skills/, which is intentionally only the Webcmd discovery skill after the dependency PR.

Testing

• uv run --with pyyaml python /Users/ankitranjan/.codex/skills/.system/plugin-creator/scripts/validate_plugin.py /Users/ankitranjan/Work/webcmd
• node -e "JSON.parse(require('fs').readFileSync('.codex-plugin/plugin.json','utf8')); JSON.parse(require('fs').readFileSync('.agents/plugins/marketplace.json','utf8')); console.log('json ok')"
• npm pack --dry-run