feat: implement comprehensive checkpoint/resume system

[TerminallyLazy]

Add complete checkpoint mechanism for saving and restoring agent state:

Core Features:

• Save/restore full agent context including history, config, and execution state
• Multiple storage backends (file-based and in-memory)
• Automatic checkpoints at configurable message intervals
• Manual checkpoint creation via tool
• Checkpoint validation with structure hash verification
• Cleanup of old checkpoints (keeps last N)

Backend Components:

• AgentCheckpoint model with Pydantic V2 validation
• CheckpointStore interface with FileCheckpointStore and MemoryCheckpointStore
• CheckpointManager singleton for centralized operations
• Integration with TaskScheduler for checkpoint-based task resumption

API Endpoints:

• POST /api_checkpoint_create - Create manual checkpoint
• GET /api_checkpoint_list - List checkpoints (supports ?all=true for all contexts)
• POST /api_checkpoint_restore - Restore from checkpoint
• DELETE /api_checkpoint_delete - Delete checkpoint
• POST /api_checkpoint_cleanup - Remove old checkpoints

UI Components:

• Checkpoint modal with context grouping and view toggle
• Support for viewing checkpoints from deleted chats
• Per-context cleanup functionality
• Quick access button in sidebar

Extensions:

• Auto-checkpoint at message intervals (configurable)
• Duplicate prevention via message count tracking

Files: 16 changed, 1839 insertions(+)