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/cli/interactive.py

@@ -1,6 +1,5 @@
 """Interactive mode for gitignore-generator."""
 
-import sys
 from typing import List, Optional, Tuple
 
 import click
@@ -130,7 +129,7 @@ def run_interactive_wizard() -> Optional[Tuple[str, str]]:
                 content_parts.append("\n")
 
         if custom_patterns:
-            content_parts.append(f"# --- Custom Patterns ---\n")
+            content_parts.append("# --- Custom Patterns ---\n")
             content_parts.extend(custom_patterns)
             content_parts.append("\n")