Function find_cef_binary 

pub(crate) fn find_cef_binary(runtime_dir: &Path) -> PathBuf

Find the CEF host binary in the runtime directory. Tries versioned name first (agentmux-X.Y.Z.exe), then the old agentmux-cef-X.Y.Z.exe pattern for backwards compat, then plain agentmux-cef.exe (dev mode).