refactor: rename mcp-builder to create-mcp-server with original content

- Rename built-in skill from 'mcp-builder' to 'create-mcp-server'
- Replace enhanced skill content with original fetch_instructions content
- Convert dynamic paths to static/generic instructions
- Remove mcp-builder reference docs, scripts, and LICENSE
- Update test to expect create-mcp-server instead of mcp-builder

The enhanced mcp-builder skill will be added in a separate PR.

Author: hannesrudolph