Laravel Boost guideline and skill #8

Ship resources/boost/guidelines/core.blade.php and resources/boost/skills/javaabu-post-types/SKILL.md (with references/ if long), so Laravel Boost picks them up from the Composer package.

The skill tells an assistant how to use the package correctly in an app: the main APIs, the rules, and the common mistakes. Follow Javaabu codestyle in every PHP example.