[ENHANCEMENT] Task as Checkpoint: Review History and Decide Next Move #854

Duplicate of #856

This issue has been merged into Task Tree Abstraction (#856).

Original Problem

Need a checkpoint mechanism to review history and decide next steps.

How Task Tree Solves This

Task Tree abstraction makes checkpoints tree node properties:

• checkpointEnabled: boolean — per-task setting
• createCheckpoint() / restoreCheckpoint(id) — methods on Task interface
• Auto-trigger when depth reaches maxDepth - 1 (optional)
• Checkpoint data includes conversationHistory + toolState + todoStatus

See #856 for full design.