Persistent checkpoint & session history

[hellodk]

Description

Enable users to save and restore plugin state across sessions.

Acceptance Criteria

• Plugin can save checkpoint snapshots of current state
• Checkpoints are persisted to disk (workspace-local storage)
• Users can view list of previous checkpoints
• Users can restore a previous checkpoint
• Session history shows timestamp and state summary
• Old checkpoints are automatically cleaned up (retention policy)

Tests Required

• Unit tests for checkpoint save/restore logic
• Integration tests for state persistence
• Unit tests for checkpoint cleanup/retention
• E2E tests for checkpoint UI interactions