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/issues/__init__.py

@@ -0,0 +1,20 @@
+"""
+Issue domain module.
+
+Contains domain models, services, and interfaces for issue management.
+"""
+
+from .models import Issue, Label, IssueState, LabelCategories
+from .services import IssueStatusService, IssueValidationService
+from .exceptions import IssueDomainError, IssueValidationError
+
+__all__ = [
+    'Issue',
+    'Label',
+    'IssueState',
+    'LabelCategories',
+    'IssueStatusService',
+    'IssueValidationService',
+    'IssueDomainError',
+    'IssueValidationError'
+]