[Personal-WP] Add my.wordpress.net specific MCP instructions and skills

[akirk]

What?

Adds MyWP-specific MCP prompts and tools through a dedicated package while keeping the generic Playground MCP server extensible.

Why?

External MCP clients need MyWP-specific operating instructions and task skills without changing the generic Playground MCP behavior.

How?

• Lets createServer accept MCP server definitions and extensions that can add prompts or register tools.
• Adds a side-effect-free @wp-playground/mcp/api entry point that exposes the generic MCP server building blocks.
• Keeps the generic Playground MCP server free of MyWP-specific prompt and tool dependencies.
• Moves MyWP-specific MCP prompts, tools, and CLI composition into a dedicated @wp-playground/personal-wp-mcp package that uses @wp-playground/mcp/api underneath.
• Exposes the MyWP MCP executable as mywp, defaulting to https://my.wordpress.net/ while still supporting --url=... for another Personal Playground origin.
• Registers MyWP prompts for agent workflow, abilities, file editing, plugin development, and syncing local changes into the MyWP sandbox.
• Adds a MyWP-only mywp_get_plugin_guidance tool that applies AI Assistant filters in the connected site and returns structured entries plus prompt-ready instructions for preferring the WordPress Abilities API for matching ability categories/domains.
• Documents the MyWP MCP configuration and local sync workflow.
• Adds unit coverage for generic prompt extension registration and MyWP plugin guidance collected from ai_assistant_ability_domains.

Testing

• npm run format:uncommitted
• npm exec nx run playground-mcp:test:unit
• npm exec nx run playground-personal-wp-mcp:test:unit
• npm exec nx run playground-mcp:lint
• npm exec nx run playground-personal-wp:lint
• npm exec nx run playground-personal-wp-mcp:lint
• npm exec nx run playground-personal-wp-mcp:build
• npm exec nx run playground-mcp:build