feat: Implement domain logic separation with clean architecture

- Created complete domain layer with pure business logic
- Implemented Issue domain models with 48 passing tests
- Implemented Project domain models with 31 passing tests
- Added domain services for complex business operations
- Established clean separation between domain, application, and infrastructure
- All 250 tests passing with no breaking changes

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

domain/projects/__init__.py

@@ -0,0 +1,18 @@
+"""
+Project domain module.
+
+Contains domain models, services, and interfaces for project management.
+"""
+
+from .models import Project, Milestone, ProjectState
+from .services import ProjectManagementService
+from .exceptions import ProjectDomainError, ProjectValidationError
+
+__all__ = [
+    'Project',
+    'Milestone',
+    'ProjectState',
+    'ProjectManagementService',
+    'ProjectDomainError',
+    'ProjectValidationError'
+]