Use Anthropic Prompt Caching

[janpawellek]

This project uses claude-sonnet-4-20250514 as the default model, which makes a lot of sense.

However, is it possible to enable Anthropic's prompt caching feature then? This would drastically reduce the cost of an agent run, as Deep Agents may have a large number of long-running tool calls.

[christian-bromann]

Will add prompt caching in #32