refactor: fix linting issues and code quality

- Fix import ordering in __init__.py
- Remove unused imports from dependencies.py
- Fix import sorting across multiple files
- Apply ruff auto-fixes

No functional changes

tests/unit/test_core/test_config.py

@@ -1,12 +1,9 @@
 """Tests for core configuration."""
 
-import os
-from unittest.mock import patch
 
 import pytest
 
 from notebooklm_agent.core.config import Settings, get_settings
-from notebooklm_agent.core.exceptions import NotebookLMAgentError, ValidationError
 
 
 @pytest.mark.unit