Function begin 

pub fn begin(
    store: &Talks,
    cfg: &Config,
    repo: PathBuf,
    agent: Option<&str>,
) -> Result<Talk>

Open a conversation. Unlike crate::chat::start this takes no agent turn: there is no idea to answer yet, and a conversation the operator has not said anything into yet is a normal, valid thing to have sitting on the phone.

agent is resolved the same way magi plan and crate::chat::start resolve their interviewer: plan::pick against [roles] planner, so this surface adds no configuration of its own.