[1.x] Add Skills support

[pushpak1300]

This PR adds Skills support to Laravel Fortify, following the official Boost convention for separating guidelines and skills.

• Simplified core.blade.php guidelines to be concise and foundational
• Added developing-with-fortify skill with detailed implementation patterns

Guidelines vs. Skills

Laravel Boost provides two distinct ways to give AI agents context about your application: guidelines and skills.

Guidelines are loaded upfront when the AI agent starts, providing essential context about package conventions and best practices that apply broadly across your codebase.

Skills are activated on-demand when working on specific tasks, containing detailed patterns for particular domains (like Livewire components or Pest tests). Loading skills only when relevant reduces context bloat and improves code quality.

Aspect	Guidelines	Skills
Loaded	Upfront, always present	On-demand, when relevant
Scope	Broad, foundational	Focused, task-specific
Purpose	Core conventions & best practices	Detailed implementation patterns