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/exceptions.py

@@ -0,0 +1,29 @@
+"""
+Domain-specific exceptions for issue management.
+"""
+
+
+class IssueDomainError(Exception):
+    """Base exception for issue domain errors."""
+
+    def __init__(self, message: str, issue_number: int = None):
+        super().__init__(message)
+        self.issue_number = issue_number
+
+
+class IssueValidationError(IssueDomainError):
+    """Exception raised when issue validation fails."""
+
+    def __init__(self, message: str, field: str = None, value=None):
+        super().__init__(message)
+        self.field = field
+        self.value = value
+
+
+class IssueStateError(IssueDomainError):
+    """Exception raised when invalid state transitions are attempted."""
+
+    def __init__(self, message: str, current_state: str, attempted_state: str):
+        super().__init__(message)
+        self.current_state = current_state
+        self.attempted_state = attempted_state