fix(agent): say why a background write was skipped

spawn_write and the volatile reapply logged "no inventory channel" for every failed resolve, but DeviceOp::resolve also fails when the host gate refuses device I/O — a laptop asleep or locked looked, in the log, like a device that had vanished from inventory.

resolve now returns Unresolved, which names the two reasons, and both background paths log it. run() still maps either to WriteError::DeviceNotFound, which is what a requester has always seen.