Support OPENAI sub

[zikajk]

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It looks that OPENAI sub is not officially supported in other coding agents:
nikomatt69/nikcli-main#77
https://x.com/thdxr/status/2009803906461905202

[ericdallo]

Done!

Now eca is doing the same as opencode: we send a specific codex prompt in system prompt (instructions) here, but append ECA's system prompt as a first user message.
The plan behavior is not great but I suspect openai will improve that prompt in the next days, agent works great, now models like gpt-5.2-codex work great !