bootstrap: clearer messaging when a push times out

Distinguish a timeout from a size rejection in the auto-batch notice ("target push timed out" vs "target rejected pack") so the user sees why the retry kicked in.

When a one-shot push hits a batchable failure (408/504/413) but no batched fallback is possible — the source can't serve the protocol-v2 fetch filter checkpointing needs — wrap the error with guidance instead of surfacing a bare "http 408", while still wrapping the original error for callers that inspect it.