Add option to auto-enable voice mode on session start

I use voice mode in nearly every session. Currently, I have to run /voice manually each time I start a new session to enable it. There's also a UX quirk where the first /voice says "Voice mode disabled" and I have to run it a second time to get "Voice mode enabled" — suggesting some stale state is carried over but not properly initialized.

Request: Add a persistent setting (e.g. "voiceAutoEnable": true in settings.json) or a CLI flag (claude --voice) that automatically activates voice mode when a session starts.

Current behavior:

1. Start new session
2. Run /voice → "Voice mode disabled"
3. Run /voice again → "Voice mode enabled"

Expected behavior:
Session starts with voice mode already enabled based on a user setting.