refactor: consolidate snapshot functionality into Store interface

The standalone Checkpointer module was designed but never integrated into
the Agent runtime. All snapshot/fork operations already use Store directly:
- Agent.snapshot() -> Store.saveSnapshot()
- Agent.fork() -> Store.loadSnapshot()

This simplifies the codebase by removing the unused abstraction layer,
making it clearer for developers which API to use.

Removed:
- src/core/checkpointer.ts (MemoryCheckpointer)
- src/core/checkpointers/ (FileCheckpointer, RedisCheckpointer)
- Related tests and exports

Updated documentation to reflect the actual API (snapshot vs checkpoint).

Author: CrazyBoyM

README.md

@@ -145,12 +145,12 @@ On crash: Resume from last safe breakpoint, auto-seal incomplete tool calls
 ### 3. Fork & Trajectory Exploration
 
 ```typescript
-// Create a checkpoint at current state
-const checkpointId = await agent.checkpoint('before-decision');
+// Create a snapshot at current state
+const snapshotId = await agent.snapshot('before-decision');
 
 // Fork to explore different paths
-const explorerA = await agent.fork(checkpointId);
-const explorerB = await agent.fork(checkpointId);
+const explorerA = await agent.fork(snapshotId);
+const explorerB = await agent.fork(snapshotId);
 
 await explorerA.chat('Try approach A');
 await explorerB.chat('Try approach B');

docs/ROADMAP.md

@@ -17,7 +17,7 @@
 | Multi-provider | Stable | Anthropic, OpenAI, Gemini, DeepSeek, Qwen, GLM... |
 | Tool System | Stable | Built-in + MCP protocol |
 | AgentPool | Stable | Up to 50 agents per process |
-| Checkpointer | Stable | Memory, File, Redis implementations |
+| Snapshot/Fork | Stable | Explore different agent trajectories |
 | Context Compression | Stable | Automatic history management |
 | Hook System | Stable | Pre/post model and tool hooks |