feat: restructure documentation and add agent guidelines

[allisson]

Split the monolithic README into focused documentation:

• Created AGENTS.md with comprehensive AI coding guidelines
• Split README into modular docs/ directory with 6 focused guides
• Improved navigation with clear linking between documents
• Added emojis and better formatting for readability

Documentation Structure:

• docs/getting-started.md - Installation and setup guide
• docs/architecture.md - Architectural patterns and design principles
• docs/development.md - Development workflow and coding standards
• docs/testing.md - Testing strategies and best practices
• docs/error-handling.md - Error handling system guide
• docs/adding-domains.md - Step-by-step guide to add new domains

AGENTS.md provides:

• Build, lint, and test commands with examples
• Clean Architecture layer responsibilities
• Code style guidelines (imports, formatting, naming)
• Error handling patterns and flows
• Validation rules and custom validators
• Database patterns and transaction management
• Testing philosophy with real database integration
• Guidelines for adding new domains
• Common pitfalls to avoid

README.md improvements:

• Concise overview with feature highlights
• Quick start section with essential commands
• Clear navigation to detailed documentation
• Better visual hierarchy with badges and emojis
• Focused on getting developers productive quickly

Domain model updates:

• Added missing package documentation comments
• Improved consistency in error definitions