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/template_loader.py

@@ -1,7 +1,6 @@
 """Template loader for gitignore-generator."""
 
 import json
-import os
 from pathlib import Path
 from typing import Dict, List, Optional, Set
 
@@ -25,6 +24,7 @@ class TemplateLoader:
                     self._templates_index = json.load(f)
             else:
                 self._templates_index = {"languages": [], "ides": [], "custom": []}
+        assert self._templates_index is not None
         return self._templates_index
 
     def _save_templates_index(self) -> None:
@@ -86,7 +86,6 @@ class TemplateLoader:
         if category:
             categories = [category]
         else:
-            index = self._load_templates_index()
             categories = ["languages", "ides"]
 
         for cat in categories: