[codex] add developer instructions

[apanasenko-oai]

we are using developer instructions for code reviews, we need to pass them in cli as well.

[pakrym-oai]

You'll need to update parse_user_message to ignore these instructions.

[apanasenko-oai]

parse_user_message

developer instructions comes with role = developer, we don't need to filter them here, right?

        ResponseItem::Message { role, content, .. } => match role.as_str() {
            "user" => parse_user_message(content).map(TurnItem::UserMessage),
            "assistant" => Some(TurnItem::AgentMessage(parse_agent_message(content))),
            "system" => None,
            _ => None,
        },

[pakrym-oai]

I don't think we need to add this to mcp server just yet.