feat: Implement unified configuration management system

Consolidates scattered configuration patterns across TDDAI, Gitea, and
MarkiTect into a unified, maintainable system addressing issue #22.

Key improvements:
- Created centralized config/ module with base classes and utilities
- Eliminated duplicate load_dotenv_file() functions
- Standardized environment variables with MARKITECT_ prefix
- Implemented comprehensive validation with helpful error messages
- Maintained full backward compatibility with existing TDDAI config

Architecture:
- BaseConfig: Abstract base with common functionality
- MarkitectConfig: Main configuration class with legacy support
- Compatibility layer: TddaiConfigCompat and GiteaConfigCompat wrappers
- Unified error handling: ConfigurationError hierarchy

All existing tests pass without modification, ensuring seamless transition.

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

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

config/exceptions.py

@@ -0,0 +1,73 @@
+"""
+Configuration-specific exceptions.
+
+Provides a unified exception hierarchy for all configuration-related errors
+across the MarkiTect project.
+"""
+
+from typing import Optional, Dict, Any
+
+
+class ConfigurationError(Exception):
+    """Base exception for configuration-related errors.
+
+    Provides enhanced error context and troubleshooting guidance.
+
+    Args:
+        message: Human-readable error description
+        field: Specific configuration field that caused the error
+        value: The invalid value (if applicable)
+        suggestion: Suggested fix for the error
+        context: Additional context information
+    """
+
+    def __init__(self,
+                 message: str,
+                 field: Optional[str] = None,
+                 value: Optional[Any] = None,
+                 suggestion: Optional[str] = None,
+                 context: Optional[Dict[str, Any]] = None):
+        super().__init__(message)
+        self.field = field
+        self.value = value
+        self.suggestion = suggestion
+        self.context = context or {}
+
+    def __str__(self) -> str:
+        """Enhanced string representation with troubleshooting guidance."""
+        parts = [super().__str__()]
+
+        if self.field:
+            parts.append(f"Field: {self.field}")
+
+        if self.value is not None:
+            parts.append(f"Value: {self.value}")
+
+        if self.suggestion:
+            parts.append(f"Suggestion: {self.suggestion}")
+
+        return " | ".join(parts)
+
+
+class ConfigValidationError(ConfigurationError):
+    """Configuration validation specific errors.
+
+    Raised when configuration values fail validation checks.
+    """
+    pass
+
+
+class ConfigFileError(ConfigurationError):
+    """Configuration file related errors.
+
+    Raised when configuration files cannot be read, parsed, or found.
+    """
+    pass
+
+
+class EnvironmentVariableError(ConfigurationError):
+    """Environment variable related errors.
+
+    Raised when required environment variables are missing or invalid.
+    """
+    pass