Add plugin scaffold generator

[PAMulligan]

Summary

Add a plugin scaffolding generator alongside the existing theme scaffold.

Details

• Generate plugin boilerplate with proper WordPress plugin headers
• Include custom post type, taxonomy, and settings page scaffolds
• Add plugin-specific agents for development automation
• Include PHPUnit test boilerplate

Acceptance Criteria

• Plugin scaffold generator creates valid WordPress plugin structure
• Custom post type and settings page templates included
• Plugin-specific agents created for development workflow