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/exceptions.py

@@ -0,0 +1,28 @@
+"""
+Domain-specific exceptions for project management.
+"""
+
+
+class ProjectDomainError(Exception):
+    """Base exception for project domain errors."""
+
+    def __init__(self, message: str, project_name: str = None):
+        super().__init__(message)
+        self.project_name = project_name
+
+
+class ProjectValidationError(ProjectDomainError):
+    """Exception raised when project validation fails."""
+
+    def __init__(self, message: str, field: str = None, value=None):
+        super().__init__(message)
+        self.field = field
+        self.value = value
+
+
+class MilestoneError(ProjectDomainError):
+    """Exception raised when milestone operations fail."""
+
+    def __init__(self, message: str, milestone_id: int = None):
+        super().__init__(message)
+        self.milestone_id = milestone_id