Function open 

pub fn open(
    store: &Chats,
    cfg: &Config,
    repo: PathBuf,
    idea: &str,
    agent: Option<&str>,
    from: Option<&Chat>,
) -> Result<Chat>

build, then persist. No first agent turn is taken.

Convenient when there is nothing racing the write - start is the only caller - but POST /api/chats cannot use it: see build’s doc for why the claim has to land between construction and this function’s own Chats::put.