feat(opencode): support custom system prompt per model

[mjdouglas]

Issue for this PR

Closes #18177

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Custom models configured in opencode.json fall back to a generic system prompt because SystemPrompt.provider() only matches known model families (claude, gemini, gpt, etc.). The agent-level prompt field doesn't solve this because agents and models are orthogonal — a model may need specific instructions regardless of which agent uses it.

This adds a prompt field to the model config. When set, it's used as the system prompt instead of the default model-family matching. Supports inline strings or file references via the existing {file:path} config substitution:

{
  "provider": {
    "my-provider": {
      "models": {
        "my-model": {
          "prompt": "You are a custom coding assistant."
        }
      }
    }
  }
}

{
  "provider": {
    "my-provider": {
      "models": {
        "my-model": {
          "prompt": "{file:./prompts/my-model.md}"
        }
      }
    }
  }
}

How did you verify your code works?

bun test test/session/system.test.ts test/provider/provider.test.ts — 79 tests pass, covering inline prompts, priority over model ID matching, fallback behavior, and config plumbing.

Screenshots / recordings

Not a UI change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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