feat(boost): ship ai guidelines and a skill

[danielebarbaro]

Adds Laravel Boost guidelines and the custom-fields-development skill.

• Guideline: model setup, defining fields, value API, querying, config keys.
• Skill: install, enabling a model, field types, forms, custom field types, filtering and sorting, replacing models, events.
• BoostResourcesTest: file shape, Blade render, size limit, frontmatter, laravel/boost only in suggest.
• Drift checks: classes, methods, trait signatures, the FieldType contract, config keys, field types, storage columns, operations, publish tags and migrations must match the code. Every config key and registered type must also be documented.
• README section and CHANGELOG entry.