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>

tddai/coverage_analyzer.py

@@ -246,8 +246,21 @@ class CoverageAnalyzer:
                     coverage_keywords=coverage_keywords,
                     related_issue=related_issue
                 ))
+            except (OSError, IOError, UnicodeDecodeError) as e:
+                # Skip files that can't be read due to file system or encoding issues
+                # Log the issue but continue processing other files
+                import logging
+                logging.getLogger(__name__).warning(
+                    f"Could not read test file {test_file}: {e}"
+                )
+                continue
             except Exception as e:
-                # Skip files that can't be read
+                # Unexpected errors should be logged but not silently ignored
+                import logging
+                logging.getLogger(__name__).error(
+                    f"Unexpected error processing test file {test_file}: {e}",
+                    exc_info=True
+                )
                 continue
 
         return existing_tests