fix(execx): give detached children the null device, not a pipe

SpawnDetached set Stdout/Stderr to io.Discard, which os/exec implements as a pipe drained by a goroutine in the parent. Once the parent exits, the child's next write raises SIGPIPE and kills it. The detached __opf_flush worker prints "scanning checkpoints…" to stderr before inference, so on a normal first push it died before rewriting anything and left no log line. Leaving the streams nil opens the null device.

Also reword the raw-byte cap error: on git-refs it is counted per checkpoint ref, not across all unpushed commits.

Entire-Checkpoint: 01M3D528TPPMAKFG69YKNRFTZD