Function base_agent_builder 

pub fn base_agent_builder(llm_client: Arc<dyn LlmClient>) -> AgentBuilder

Returns an AgentBuilder with sensible defaults:

• English
• Medium reasoning effort
• Thinking enabled
• Consecutive failure recovery (default 3 retries)
• Session limits (50/100/50k)

Callers are responsible for: registering tools, setting the approval handler, setting the system prompt, then calling .build().