feat: add FileSDK module for file storage operations

[NianJiuZst]

Summary

Closes #142

Adds a file property to MiniMaxSDK so SDK users can manage files in MiniMax storage without manually constructing HTTP requests.

Motivation

Every CLI resource has a corresponding SDK module — except file storage:

CLI command	SDK method	Status
mmx file upload	sdk.file.upload()	Added
mmx file list	sdk.file.list()	Added
mmx file delete	sdk.file.delete()	Added
(no CLI command)	sdk.file.retrieve()	Added

Usage

import { MiniMaxSDK } from 'mmx-cli/sdk';

const sdk = new MiniMaxSDK({ apiKey: 'sk-...', region: 'global' });

// Upload
const { file } = await sdk.file.upload('/path/to/doc.pdf', 'retrieval');

// List
const { files } = await sdk.file.list();

// Get metadata
const info = await sdk.file.retrieve(file.file_id);

// Delete
await sdk.file.delete(file.file_id);

Implementation

• FileSDK extends Client and reuses existing file*Endpoint() functions and API types from the shared client layer
• upload() mirrors the CLI's FormData construction and file existence validation
• All four file endpoints (/v1/files, /v1/files/list, /v1/files/delete, /v1/files/retrieve) are covered

Files changed

File	Change
src/sdk/file/index.ts	New file — FileSDK class (70 lines)
src/sdk/index.ts	+3 lines — import and expose file property
test/sdk/file.test.ts	New file — file existence and upload smoke tests

Screenshots