Add more presets to the system prompt

Currently, the only preset is [claude_code](https://code.claude.com/docs/en/agent-sdk/modifying-system-prompts), I do not want to use the `claude_code` preset, but I do want to be able to use parts of it, for example the part that instructs the auto memory.

It is also pretty unclear what is in the `claude_code` preset, to give a good understanding of what should be added to a custom prompt to get certain behavior to work correctly (such as auto memory).

My proposal would be to split up parts of the `claude_code` preset into smaller presets and the ability to construct your own, for example:

```
system_prompt={
"type": "preset",
"presets": ["formatting_guidelines", "auto_memory", "safety_instructions"],
"append": "You are a custom agent"
}
```