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/index.ts

@@ -248,3 +248,14 @@ export type {
   FileIndexEntry,
   FileSearchOptions,
 } from './file-index/index.js';
+
+// Constants - 统一存储路径
+export {
+  APP_DIR_NAME,
+  getAppDir,
+  getConfigDir,
+  getDataDir,
+  getCheckpointsDir,
+  getCacheDir,
+  getLogsDir,
+} from './constants/index.js';