Function builtin_spawn_agent 

pub async fn builtin_spawn_agent(
    args: &[DslValue],
    ctx: &ReasoningBuiltinContext,
) -> Result<DslValue>

Execute the spawn_agent builtin: register a new named agent.

Arguments (named via map or positional):

• name: string — agent name
• system: string — system prompt
• tools: list of strings (optional)
• response_format: string (optional)

Returns a map with agent_id and name.