repo mirror: only diagnose suspended mirror on existing placement

A fresh create that races into invalid_target is propagation lag, not suspension — a mirror created moments ago cannot be suspended. Gate the suspended-mirror diagnosis on a non-fresh create so we don't misdirect users to a mirrors resume command that does nothing; let the raw error surface instead. The precondition is enforced inside explainSuspendedMirror so it can't be missed by a future caller.