[Feature] Allow configuring / adding additional developer instructions for the new voice transcription feature

What variant of Codex are you using?

v0.106.0-alpha.8 CLI

What feature would you like to see?

Re: https://github.com/openai/codex/pull/3381 the new voice_transcription flag for voice transcription

The voice transcription API allows one to send a short prompt along with the audio for context. Please allow customizing (or adding to) the prompt so that we can include additional instructions. This would be useful in many cases, especially where one might speak in a foreign language but would want the transcription to always be done in English. It'll also be useful to those that work in a certain 'domain' and would want their transcriptions to infer / fill-in-the-blanks where the audio was deliberately terse. I've separately had great success with this using the API.

Additional information

_No response_