#20340: feat(tts): add instructions field to OpenAI TTS provider

Add support for the `instructions` field in the OpenAI TTS provider config, enabling voice personality/style control. ```json { "messages": { "tts": { "provider": "openai", "openai": { "model": "gpt-4o-mini-tts", "voice": "onyx", "instructions": "Speak in a calm, measured tone with slight pauses before key information." } } } } ``` ## Context The OpenAI `/v1/audio/speech` API supports an `instructions` parameter that controls voice affect, personality, pacing, accent, and pronunciation. This field was not previously exposed in the OpenClaw config.