refactor(core): 统一存储路径到 ~/.ai-terminal-assistant/

- 新增 constants/paths.ts 模块统一管理存储路径
- Session 数据从 ~/.local/share/ai-assist/ 迁移到 ~/.ai-terminal-assistant/data/
- Checkpoint 从 ~/.ai-assist/checkpoints/ 迁移到 ~/.ai-terminal-assistant/checkpoints/
- MCP 配置从 ~/.ai-assist/config.* 迁移到 ~/.ai-terminal-assistant/mcp.*
- Agent 配置保持在 ~/.ai-terminal-assistant/agents.json

packages/core/src/checkpoint/manager.ts

@@ -4,8 +4,8 @@
  */
 
 import * as path from 'path';
-import * as os from 'os';
 import { nanoid } from 'nanoid';
+import { getCheckpointsDir } from '../constants/paths.js';
 import { ShadowGit, createShadowGit } from './shadow-git.js';
 import { CheckpointLock } from './lock.js';
 import { CheckpointSafetyChecker } from './safety.js';
@@ -75,7 +75,7 @@ export class CheckpointManager {
       },
       maxCheckpoints: 100,
       maxAge: 7 * 24 * 60 * 60 * 1000,
-      storageDir: path.join(os.homedir(), '.ai-assist', 'checkpoints'),
+      storageDir: getCheckpointsDir(),
       ...config,
     };