Move all attribute-related code from lib/treaty/attribute to lib/treaty/entity/attribute
to better reflect the domain model where attributes belong to entities, not as a
separate top-level concern.

Changes:
- Move attribute builder, options, validators, and validation orchestration classes
- Move attribute base class, DSL, collection, and helper utilities
- Move conditional logic (if/unless) for attribute options
- Move all attribute option modifiers (as, cast, computed, default, transform)
- Move attribute validators (format, inclusion, required, type)
- Add new entity builder to coordinate attribute initialization
- Update request and response attribute classes to reference new location
- Update request and response validators, factories, and entities
- Update core engine and entity module to use reorganized structure
- Move corresponding test suite from spec/treaty/attribute to spec/treaty/entity/attribute
- Update documentation for core concepts and entities

This reorganization improves code cohesion by grouping attribute logic with
its containing entity, making the architectural intent clearer.