refactor(storage): 采用 OpenCode 风格三层存储结构

重构消息存储系统，从"每条消息一个文件"改为分层存储：
- Session → Message → Parts 三层结构
- 12 种 Part 类型（TextPart, ToolPart, ReasoningPart 等）
- ToolPart 状态机（pending → running → completed/error）
- 通用 Storage API（read/write/list/remove）

新增文件:
- parts.ts: Part 类型定义（Zod schema）
- message.ts: MessageInfo 类型定义
- id.ts: ID 生成器
- storage/: 分层存储实现

删除旧文件:
- storage.ts, types.ts, migration.ts

存储路径:
~/.local/share/ai-assist/
├── session/{projectId}/{sessionId}.json
├── message/{sessionId}/{messageId}.json
├── part/{messageId}/{partId}.json
└── todo/{sessionId}.json

packages/core/src/index.ts

@@ -14,9 +14,30 @@ export type {
   CompressionConfig,
   DetailedCompressionResult,
 } from './context/index.js';
-export { SessionStorage } from './session/storage.js';
+// Session - 新的三层存储结构
 export { SessionManager } from './session/index.js';
-export type { SessionData, SessionSummary } from './session/types.js';
+export type { SessionData, SessionSummary, ProjectMetadata } from './session/index.js';
+
+// Session Storage API
+export {
+  SessionStorage,
+  MessageStorage,
+  PartStorage,
+  TodoStorage,
+  initStorage,
+  getStorageDir,
+  StorageNotFoundError,
+} from './session/index.js';
+
+export type {
+  SessionInfo,
+  MessageInfo,
+  Part,
+  PartType,
+  ToolStatus,
+  TodoItem,
+  TodoList,
+} from './session/index.js';
 
 // Types
 export type { UserInput, ChatResult } from './types/index.js';