Function handoff_tool_spec 

pub fn handoff_tool_spec(
    target: &str,
    description: Option<&str>,
) -> ToolDefinition

Build the marker tool spec for handing off to target. Rust analogue of Python’s _create_handoff_tool.

The returned ToolDefinition is a non-executable marker (no local executor): the coordinator intercepts calls to it rather than running it. Attach it to an agent’s tools so its model can request the transfer.