Fail snapshot checkpoints on redaction errors instead of dropping the transcript

CondenseSession drops the transcript and continues when runtime redaction fails, which is right for the hook paths: a commit must never be blocked on it. For checkpoint create it meant printing an ID for a checkpoint written without its transcript, or reporting "nothing to checkpoint" when the session had no files.

Add condenseOpts.failOnRedactionError, set only by CreateSnapshotCheckpoint: redactOrDrop returns the failure and nothing is written. Hook-path behavior is unchanged.