Function begin_with 

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

begin with remembered choices. An explicit agent wins over preferred.agent and keeps its strict error. A preferred agent that is on the roster and runnable is an explicit choice (fallback = false) unless it is the chatter chain’s own head. Persona and implementers are applied before the single save, so neither is marked dirty: the new seat’s briefing already carries them.