Switch from system prompt to instructions

[simonw]

I'm currently using the old system prompt messages:

llm-openai-plugin/llm_openai.py

Lines 161 to 176 in e0e8d7d

	def _build_messages(self, prompt, conversation):
	messages = []
	current_system = None
	image_detail = None
	if self.vision:
	image_detail = prompt.options.image_detail or "low"
	if conversation is not None:
	for prev_response in conversation.responses:
	if (
	prev_response.prompt.system
	and prev_response.prompt.system != current_system
	):
	messages.append(
	{"role": "system", "content": prev_response.prompt.system}
	)
	current_system = prev_response.prompt.system

Should switch that over to using "instructions" instead: https://platform.openai.com/docs/api-reference/responses/create#responses-create-instructions