feat: add depth option to clone/fetch for shallow clones

[mizchi]

Summary

• Expose existing protocol-level deepen support to the JavaScript API
• Add depth?: number to BitCloneOptions and BitFetchOptions
• When depth is 0 (default), full clone behavior is preserved
• When depth > 0, the server returns only the specified number of commits

Changes

• src/lib/js_api_exports.mbt: Add depth : Int parameter to js_clone_remote, js_fetch_remote, and their promise wrappers
• npm/lib.js: Pass options.depth ?? 0 to raw clone/fetch calls
• npm/lib.d.ts: Add depth?: number to option interfaces

Usage

import { clone, createFetchTransport } from "@mizchi/bit";

// Full clone (default, depth=0)
await clone(backend, "/repo", url, transport);

// Shallow clone (depth=1)
await clone(backend, "/repo", url, transport, { depth: 1 });

Test plan

• Full clone still works (depth=0, default)
• Shallow clone with depth=1 returns single commit
• Fetch with depth works similarly

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