Replace separate system_prompt and append_system_prompt fields with a
single system_prompt field that accepts:
- string: custom system prompt
- {"preset": "claude_code"}: use default Claude Code prompt
- {"preset": "claude_code", "append": "..."}: default prompt with
additions
- None/undefined: vanilla Claude with no system prompt
This matches the TypeScript SDK API design and provides more flexible
system prompt configuration.
🤖 Generated with [Claude Code](https://claude.ai/code)
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Co-authored-by: Claude <noreply@anthropic.com>