Normalize File Deletion Logic

Description:
Currently, file deletion fails if the user does not type .txt manually. Make the delete logic smart enough to:

Check and append .txt if not already present.
Allow deletion by typing just the base name.
Expected Behavior:

Enter file name to delete: notes
→ Deletes notes.txt (if exists)

Look for the file deletion function in the codebase, likely in a main CLI script. The logic should check if the input filename ends with '.txt'; if not, append it before attempting deletion. Test by running the program and trying to delete a file with and without the extension.

索引モデルが issue の本文から書いたものです。