checkpoint: tier the read surface into CheckpointReader / SessionReader

Mirror the write-side session/checkpoint split on reads: replace the mixed PersistentReader/PersistentListReader with CheckpointReader { Read, List } (checkpoint-level) SessionReader { ReadSession{Content,Metadata,Prompts,MetadataAndPrompts} } (session-level) and compose PersistentStore = CheckpointReader + SessionReader + Writer.

Helpers now take the minimal tier they need (ReadCheckpoint: CheckpointReader, ReadLatestSessionContent: SessionReader); callers and stubs updated. Also drops a now-dead type assertion in readSessionMetadataForExport (SessionReader guarantees ReadSessionMetadata). No behavior change.