refactor: Factor out Gitea interfacing into clean facade pattern

- Create new gitea/ package with clean API facade
- Establish proper separation of concerns: tddai uses gitea, not vice versa
- Replace duplicate curl+subprocess patterns with unified HTTP client
- Add rich domain models with properties (issue.priority, issue.status)
- Maintain full backwards compatibility in tddai modules
- Reduce code complexity: -373 lines, +151 lines (net -222 lines)
- Improve testability and maintainability through clean interfaces

Architecture:
- gitea.client.GiteaClient - main facade with sub-clients
- gitea.api_client - high-level API with model conversion
- gitea.http_client - low-level HTTP operations
- gitea.models - rich domain objects (Issue, Milestone, Label)
- gitea.config - gitea-specific configuration
- gitea.exceptions - clean exception hierarchy

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

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

gitea/exceptions.py

@@ -0,0 +1,31 @@
+"""
+Gitea-specific exceptions.
+"""
+
+
+class GiteaError(Exception):
+    """Base exception for Gitea API operations."""
+    pass
+
+
+class GiteaAuthError(GiteaError):
+    """Raised when authentication fails or token is missing."""
+    pass
+
+
+class GiteaNotFoundError(GiteaError):
+    """Raised when requested resource is not found."""
+    pass
+
+
+class GiteaApiError(GiteaError):
+    """Raised when API returns an error response."""
+
+    def __init__(self, message: str, status_code: int = None):
+        super().__init__(message)
+        self.status_code = status_code
+
+
+class GiteaConfigError(GiteaError):
+    """Raised when Gitea configuration is invalid or missing."""
+    pass