[FEATURE]: Allow disabling or replacing the built-in provider system prompt #39537

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Describe the enhancement you want to request

OpenCode selects a system prompt based on model family (anthropic.txt, gemini.txt, etc.) with no config option to disable or replace it.

This contradicts the tool's openness — anyone wanting full control over what gets sent to the model (custom personas, stripped-down prompts, compliance requirements) has no recourse short of patching the source.

A config option like "system_prompt": false or "system_prompt": "path/to/custom.txt" at the agent level would address this. The existing agent.prompt field is append-only.

Related: #34774 goes the opposite direction (protecting the built-in from accidental replacement).