feat: implement tddai Python library for TDD workspace management

- Create comprehensive tddai package with workspace, issue fetcher, and test generator modules
- Add Python CLI interface (tddai_cli.py) to replace complex Makefile shell logic
- Update Makefile targets to use Python CLI for better maintainability
- Implement proper behavior-based tests instead of file existence checks
- Add workspace lifecycle management (create, active, finish, cleanup)
- Add issue fetching from Gitea API with error handling
- Add comprehensive test coverage with 19 passing tests
- Support environment variable configuration for different deployments

This addresses issue #11: Setup TDD workspace infrastructure
All tests pass and the system achieves green state before commit.

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

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

tddai/exceptions.py

@@ -0,0 +1,28 @@
+"""
+Custom exceptions for tddai library.
+"""
+
+
+class TddaiError(Exception):
+    """Base exception for all tddai errors."""
+    pass
+
+
+class WorkspaceError(TddaiError):
+    """Raised when workspace operations fail."""
+    pass
+
+
+class IssueError(TddaiError):
+    """Raised when issue operations fail."""
+    pass
+
+
+class ConfigurationError(TddaiError):
+    """Raised when configuration is invalid."""
+    pass
+
+
+class TestGenerationError(TddaiError):
+    """Raised when test generation fails."""
+    pass