feat: Enhance tddai configuration with auto-loading .env files

Configuration System Improvements:
- Add automatic .env.tddai file loading without external dependencies
- Implement load_dotenv_file() helper for lightweight env file parsing
- Maintain configuration hierarchy: Environment → .env.tddai → Defaults
- Zero breaking changes - existing setup script approach still works

Documentation:
- Create comprehensive CONFIG.md with configuration management guide
- Document hierarchy, options, platform examples, and troubleshooting
- Include migration instructions and best practices
- Cover both auto-loading and manual configuration methods

Benefits:
- Users no longer need to manually source setup scripts
- Project-agnostic configuration system remains flexible
- Improved developer experience with seamless config loading
- Complete documentation for configuration management

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

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

tddai/config.py

@@ -26,6 +26,19 @@ from dataclasses import dataclass
 from .exceptions import ConfigurationError
 
 
+def load_dotenv_file(env_file: Path) -> None:
+    """Load environment variables from a .env file."""
+    if not env_file.exists():
+        return
+
+    with open(env_file, 'r') as f:
+        for line in f:
+            line = line.strip()
+            if line and not line.startswith('#') and '=' in line:
+                key, value = line.split('=', 1)
+                os.environ.setdefault(key.strip(), value.strip())
+
+
 @dataclass
 class TddaiConfig:
     """Configuration settings for tddai."""
@@ -58,7 +71,11 @@ class TddaiConfig:
 
     @classmethod
     def from_environment(cls) -> "TddaiConfig":
-        """Create config from environment variables."""
+        """Create config from environment variables and .env files."""
+        # Auto-load .env.tddai file if it exists
+        env_file = Path(".env.tddai")
+        load_dotenv_file(env_file)
+
         config = cls()
 
         # Override with environment variables if present