hermes - 💡(How to fix) Fix Feature request: Add automatic cleanup for audio_cache files

Problem

The ~/.hermes/audio_cache/ directory stores voice files (.ogg) from TTS and STT. There is no automatic cleaning of old files, so it grows indefinitely (some are already 2+ months old), wasting disk space on the server.

Proposed changes

1. Add a config setting like audio_retention_days: 7 to the cache: or tts: section in config.yaml.
2. Implement automatic cleanup logic at gateway startup or via cron.
3. Optionally add a manual cleanup command (e.g. /purge-audio).