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/prompt_optimizer.py

@@ -14,7 +14,6 @@ import re
 from dataclasses import dataclass
 from typing import Any
 
-
 # Abbreviation mapping for common terms
 ABBREVIATIONS = {
     "price": "P",