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/compat.py

@@ -0,0 +1,169 @@
+"""
+Backward compatibility layer for existing configuration systems.
+
+Provides compatibility shims that allow existing TDDAI and Gitea
+configuration code to work with the unified configuration system.
+"""
+
+from dataclasses import dataclass
+from pathlib import Path
+from typing import Dict, Any, Optional
+
+from .manager import get_unified_config
+from .exceptions import ConfigurationError
+
+
+@dataclass
+class TddaiConfigCompat:
+    """TDDAI configuration compatibility layer.
+
+    Provides the same interface as the original TddaiConfig but
+    backed by the unified configuration system.
+    """
+
+    workspace_dir: Path
+    current_issue_file: str = "current_issue.json"
+    gitea_url: str = ""
+    repo_owner: str = ""
+    repo_name: str = ""
+    tests_dir: Path = Path("tests")
+    test_file_pattern: str = "test_issue_{issue_num}_{scenario}.py"
+    claude_code_command: str = "claude"
+
+    def __init__(self, **kwargs):
+        """Initialize from unified configuration."""
+        unified = get_unified_config()
+
+        # Map unified config to TDDAI format
+        self.workspace_dir = unified.workspace_dir
+        self.gitea_url = unified.gitea_url
+        self.repo_owner = unified.repo_owner
+        self.repo_name = unified.repo_name
+        self.tests_dir = unified.tests_dir
+        self.test_file_pattern = unified.test_file_pattern
+        self.claude_code_command = unified.claude_code_command
+
+        # Apply any explicit overrides
+        for key, value in kwargs.items():
+            if hasattr(self, key):
+                setattr(self, key, value)
+
+    def validate(self) -> None:
+        """Validate configuration (for compatibility)."""
+        if not self.gitea_url or not self.gitea_url.strip():
+            raise ConfigurationError("gitea_url cannot be empty")
+
+        if not self.repo_owner or not self.repo_owner.strip():
+            raise ConfigurationError("repo_owner cannot be empty")
+
+        if not self.repo_name or not self.repo_name.strip():
+            raise ConfigurationError("repo_name cannot be empty")
+
+    @property
+    def issues_api_url(self) -> str:
+        """Get issues API URL (for compatibility)."""
+        return f"{self.gitea_url}/api/v1/repos/{self.repo_owner}/{self.repo_name}/issues"
+
+    @property
+    def issues_base_url(self) -> str:
+        """Get issues base URL (for compatibility)."""
+        return f"{self.gitea_url}/{self.repo_owner}/{self.repo_name}/issues"
+
+
+@dataclass
+class GiteaConfigCompat:
+    """Gitea configuration compatibility layer.
+
+    Provides the same interface as the original GiteaConfig but
+    backed by the unified configuration system.
+    """
+
+    base_url: str
+    repo_owner: str
+    repo_name: str
+    auth_token: str = ""
+    api_version: str = "v1"
+
+    def __init__(self, **kwargs):
+        """Initialize from unified configuration."""
+        unified = get_unified_config()
+
+        # Map unified config to Gitea format
+        self.base_url = unified.gitea_url
+        self.repo_owner = unified.repo_owner
+        self.repo_name = unified.repo_name
+        self.auth_token = unified.api_token
+
+        # Apply any explicit overrides
+        for key, value in kwargs.items():
+            if hasattr(self, key):
+                setattr(self, key, value)
+
+    @classmethod
+    def from_tddai_config(cls, tddai_config) -> 'GiteaConfigCompat':
+        """Create from TDDAI config (for compatibility)."""
+        return cls(
+            base_url=tddai_config.gitea_url,
+            repo_owner=tddai_config.repo_owner,
+            repo_name=tddai_config.repo_name
+        )
+
+    def validate(self) -> None:
+        """Validate configuration (for compatibility)."""
+        if not self.base_url or not self.base_url.strip():
+            raise ConfigurationError("base_url cannot be empty")
+
+        if not self.repo_owner or not self.repo_owner.strip():
+            raise ConfigurationError("repo_owner cannot be empty")
+
+        if not self.repo_name or not self.repo_name.strip():
+            raise ConfigurationError("repo_name cannot be empty")
+
+        # URL validation
+        if not (self.base_url.startswith('http://') or self.base_url.startswith('https://')):
+            raise ConfigurationError("base_url must start with http:// or https://")
+
+    @property
+    def api_base_url(self) -> str:
+        """Get API base URL."""
+        return f"{self.base_url}/api/{self.api_version}"
+
+    @property
+    def repo_api_url(self) -> str:
+        """Get repository API URL."""
+        return f"{self.api_base_url}/repos/{self.repo_owner}/{self.repo_name}"
+
+    @property
+    def issues_api_url(self) -> str:
+        """Get issues API URL."""
+        return f"{self.repo_api_url}/issues"
+
+
+def get_tddai_config(**kwargs) -> TddaiConfigCompat:
+    """Get TDDAI-compatible configuration.
+
+    This function can be used as a drop-in replacement for the original
+    get_config() function in tddai/config.py.
+
+    Args:
+        **kwargs: Override values for configuration fields
+
+    Returns:
+        TDDAI-compatible configuration object
+    """
+    return TddaiConfigCompat(**kwargs)
+
+
+def get_gitea_config(**kwargs) -> GiteaConfigCompat:
+    """Get Gitea-compatible configuration.
+
+    This function can be used as a drop-in replacement for the original
+    GiteaConfig creation.
+
+    Args:
+        **kwargs: Override values for configuration fields
+
+    Returns:
+        Gitea-compatible configuration object
+    """
+    return GiteaConfigCompat(**kwargs)