fix: resolve CI lint and build failures

- Fixed lint path in CI workflow from src/gdiffer/ to gitignore_generator/
- Fixed verification command from gdiffer --version to gitignore --version
- Removed unused imports (pathlib.Path, typing.List, os, sys, pytest, unittest.mock.patch)
- Removed unused variable assignments (category, index)
- Fixed f-strings without placeholders
- Renamed ambiguous variable 'l' to 'line' and 'prev_line'
- Added type assertion for type narrowing in template_loader.py

gitignore_generator/tests/conftest.py

@@ -3,6 +3,5 @@
 import sys
 from pathlib import Path
 
-import pytest
 
 sys.path.insert(0, str(Path(__file__).parent.parent))