Function define_agent 

pub async fn define_agent<F>(build: F) -> RuntimeResult<Agent>
where
    F: FnOnce(&mut AgentBuilder) -> RuntimeResult<()>,

Build an Agent from a closure that configures an AgentSpec-like profile, mirroring Flue’s defineAgent(() => ({ ... })).

In MVP this resolves the sandbox and wires defaults; tool/skill resolution from the registry arrives in a later phase.