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/test_template_loader.py

@@ -1,6 +1,5 @@
 """Tests for template loader."""
 
-import os
 import tempfile
 from pathlib import Path
 
@@ -107,6 +106,6 @@ class TestTemplateLoader:
     def test_duplicate_removal_in_merge(self, loader_with_templates):
         """Test that duplicates are removed when merging."""
         merged = loader_with_templates.merge_templates(["python", "python"])
-        lines = [l for l in merged.splitlines() if l.strip() and not l.strip().startswith("#")]
+        lines = [line for line in merged.splitlines() if line.strip() and not line.strip().startswith("#")]
         unique_lines = set(lines)
         assert len(lines) == len(unique_lines)