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Describe the enhancement you want to request

System prompts for Anthropic, Codex, and Gemini models are very different, in ways that might not have good justification with recent models. Some if it, like Gemini system prompt, seem to be just inheritance from prior work (e.g. imported from gemini-cli).

This makes all of them diverge in guidance: some have rules about OpenCode help and support, others don't, different tone, different tool usage guidance, etc.

I have been using a fork with OpenAI/Anthropic/Google models all using the default anthropic.txt system prompt. They all work just fine with it. In fact, the gemini.txt has some verifiable issues (see #10438).

So, to improve mainteinance and consistence across providers, I'm thinking a single baseline system prompt should be used for all providers. Where some divergences are demonstrably needed (e.g. via benchmark or any decent set of reproducible test cases), a lightweight templating system could be applied to have some conditionals (e.g. make some section conditional to certain provider, or certain tool being available).