checkpoint: extract shared getSessionTree for session-level reads

The four SessionReader methods (ReadSessionMetadata, ReadSessionPrompts, ReadSessionMetadataAndPrompts, ReadSessionContent) each repeated the same prologue: context check, getFetchingTree, navigate to the checkpoint tree, then the session subtree, with identical ErrCheckpointNotFound wrapping.

Extract that navigation into getSessionTree so each reader is just "resolve session tree, read the files it needs." Behavior is unchanged — the helper returns the same ErrCheckpointNotFound (wrapping "session N not found" when the session subtree is missing).