refactor(checkpoint-remote): keep prePush and status sync under the complexity limit

The withhold branches pushed ManualCommitStrategy.prePush to cognitive complexity 34 and computeCheckpointSyncInfoCounting to 32, over the gate's 30. Each branch moves into a helper with no behavior change:

• skipsCheckpointPush folds prePush's push_sessions-off and withheld early returns into one call, and still warns on every withheld push.
• dedicatedCheckpointStoreInUse holds status's choice between the read probe and PushURL, and reports when info is already complete.