Extract checkpoint and turn-persistence logic from AgentLoop into two
dedicated modules to reduce loop.py's size and improve reviewability.
- checkpoint.py: CheckpointManager handles runtime checkpoints, pending
user-turn markers, and session recovery after cancellation/crash.
- turn_writer.py: TurnWriter handles message sanitization, turn saving,
early user-message persistence, and subagent-followup persistence.
AgentLoop retains thin delegate methods for full backward compatibility.
Test helpers updated to initialize the new sub-managers.
Rename the boolean flag in _sanitize_persisted_blocks and alias the imported helper so session persistence cannot crash with TypeError when truncation is enabled.
