clean: dead code removal + unused imports

- executor: remove dead subtitle_writer stub (lines 121-125)
- executor: replace getattr with direct field access on ResolvedVoice
- service: remove duplicate split_pattern import
- service: remove unused import time
- planner: remove unused import os
- adapters: remove unused import threading, time

abogen/pyqt/conversion_adapter.py

@@ -14,8 +14,6 @@ Subtitle file/timestamp special paths remain in ConversionThread.run() early ret
 from __future__ import annotations
 
 import os
-import threading
-import time
 from pathlib import Path
 from typing import Any, Callable, Dict, List, Optional