Modernize pf-react plugin: dual plugin dirs, rename agent, fix docs

[jpuzz0]

Cleans up the pf-react plugin and scaffolds three new plugins for workshop contributors.

1. Modernize pf-react plugin

• Add .cursor-plugin/ directories alongside .claude-plugin/ (identical content)
• Rename test-generator → unit-test-generator for clarity
• Delete redundant commands/ directory (just routed to agents)
• Align mocking advice across both agents: mock at the network boundary, not component boundaries
• Remove references to unsupported tools (Copilot, ChatGPT)
• Update all docs with tool-agnostic language
• Add CONTRIBUTING-SKILLS.md with step-by-step guide for skill contributors

2. Scaffold new plugins, add unit-test skill, remove stale cursor rule

• Scaffold pf-design-tokens, pf-a11y, and pf-figma plugins with plugin.json and README
• Wire PatternFly MCP server into all new plugins
• Register new plugins in both marketplace manifests
• Add unit-test-generator skill to pf-react (first skill in the repo)
• Remove MCP server boilerplate from existing agents (auto-discovered via plugin.json)
• Delete .cursor/rules/patternfly-vibe-coding.mdc (replaced by plugin agents)
• Update root README and CLAUDE.md to reflect current structure

Closes #10