fn forward_for_second_instance(
data_dir: &Path,
dir_hash: &str,
) -> Option<ForwardError>Expand description
What a launcher that lost the single-instance race does: open a window in
the running instance, unless it was started by a login entry, which only
asks that AgentMux be running (SPEC_START_WITH_OS_2026_09_25.md §3.6).
Not used by the macOS reopen handler, which is a user asking for a window.
Returns the forward’s error, if any, for await_running_instance.