refactor: Standardize error handling patterns across codebase

Comprehensive error handling improvements addressing inconsistent patterns:

• Created markitect/exceptions.py with complete domain-specific exception hierarchy
  - MarkitectError base class with context and cause chaining support
  - Specific exceptions for Document, AST, Cache, Database, Schema operations
  - Built-in logging and context preservation

• Fixed overly broad exception handling in tddai modules:
  - issue_fetcher.py: Replace generic Exception with specific Gitea errors
  - project_manager.py: Proper error translation with context preservation
  - coverage_analyzer.py: Replace silent suppression with logging

• Enhanced cache_service.py error handling:
  - Specific OSError/PermissionError handling for file operations
  - Logging integration for unexpected errors
  - Preserved error collection and reporting

• Implemented proper exception chaining patterns:
  - All error translations use `raise ... from e` for debugging
  - Preserved original exception context and stack traces
  - Added docstring declarations of raised exceptions

• Benefits:
  - Eliminates silent error suppression and debugging black holes
  - Provides specific, actionable error messages
  - Preserves full error context for troubleshooting
  - Establishes consistent patterns for future development

Resolves issue #21: Error handling standardization

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

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

markitect/cache_service.py

@@ -162,8 +162,15 @@ class CacheDirectoryService:
             try:
                 cache_file.unlink()
                 removed_count += 1
-            except Exception as e:
+            except (OSError, PermissionError) as e:
                 errors.append(f"Could not remove {cache_file}: {e}")
+            except Exception as e:
+                # Log unexpected errors but continue cleanup
+                import logging
+                logging.getLogger(__name__).warning(
+                    f"Unexpected error removing cache file {cache_file}: {e}"
+                )
+                errors.append(f"Unexpected error removing {cache_file}: {e}")
 
         if errors:
             return {
@@ -212,10 +219,22 @@ class CacheDirectoryService:
                 'file_removed': True,
                 'cache_file': str(cache_file)
             }
-        except Exception as e:
+        except (OSError, PermissionError) as e:
             return {
                 'success': False,
-                'message': f'Error removing cache for {source_path.name}: {e}',
+                'message': f'File system error removing cache for {source_path.name}: {e}',
+                'file_removed': False,
+                'error': str(e)
+            }
+        except Exception as e:
+            import logging
+            logging.getLogger(__name__).error(
+                f"Unexpected error removing cache for {source_path.name}: {e}",
+                exc_info=True
+            )
+            return {
+                'success': False,
+                'message': f'Unexpected error removing cache for {source_path.name}: {e}',
                 'file_removed': False,
                 'error': str(e)
             }