Add frontend-design skill to the Cowork plugin marketplace

## Description

The `frontend-design` plugin currently only exists in the [`anthropics/claude-code`](https://github.com/anthropics/claude-code/tree/main/plugins/frontend-design) repo and is only installable via the Claude Code CLI. It is not available in Cowork sessions.

Given that Cowork is a primary way users interact with Claude for building things — including frontend interfaces — the `frontend-design` skill should be available in the Cowork plugin marketplace alongside the existing 11 knowledge-work plugins.

## Why This Matters

Cowork users who build frontend applications (React, React Native, Vue, etc.) currently get no design-quality guidance from Claude in Cowork sessions, even though the same guidance is available in Claude Code via the `frontend-design` plugin. This creates an inconsistent experience across Anthropic products.

The skill itself is a single `SKILL.md` file, so adding it to this marketplace would be straightforward.

## Proposed Solution

Add a `frontend-design` plugin to this repo, either:

1. **Copy** the skill from [`anthropics/claude-code/plugins/frontend-design`](https://github.com/anthropics/claude-code/tree/main/plugins/frontend-design) into this marketplace, or
2. **Unify** the plugin systems so Claude Code plugins are automatically available in Cowork

## Reference

- Plugin source: https://github.com/anthropics/claude-code/tree/main/plugins/frontend-design
- Skill file: https://github.com/anthropics/claude-code/blob/main/plugins/frontend-design/skills/frontend-design/SKILL.md