fix: resolve linting issues in token efficiency implementation

- Fix ambiguous variable names (l → layer)
- Remove unused imports and variables
- Organize import statements

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/brain/gemini_client.py

@@ -19,9 +19,9 @@ from typing import Any
 
 from google import genai
 
-from src.config import Settings
 from src.brain.cache import DecisionCache
 from src.brain.prompt_optimizer import PromptOptimizer
+from src.config import Settings
 
 logger = logging.getLogger(__name__)