feat: add Username domain type for Linux system usernames

Add new domain type to replace hardcoded username strings:
- Username validation following Linux naming requirements
- Generic constructor accepting Into<String> for better ergonomics
- Explicit error handling with UsernameError enum
- Comprehensive test suite with 27 username-specific tests
- Support for serde serialization/deserialization
- Complete trait implementations (Display, FromStr, AsRef, etc.)

This provides type safety and validation for system usernames used in
deployment and infrastructure management.

Author: josecelano

src/domain/mod.rs

@@ -6,8 +6,11 @@
 //! ## Components
 //!
 //! - `template` - Core template domain models and business logic
+//! - `username` - Linux system username validation and management
 
 pub mod template;
+pub mod username;
 
 // Re-export commonly used domain types for convenience
 pub use template::{TemplateEngine, TemplateEngineError, TemplateManager, TemplateManagerError};
+pub use username::{Username, UsernameError};