Ambiguous error tracker

Sticky issue to track all ambiguous/unhelpful errors, add more as comments

Please keep comments for sub-issues in this format:

> * ` for `
>
> ```sh-session
> $ wkg [...]
> Error:
>
> Caused by:
> 0:
> ```

### Errors

* interface not found for fetch

```sh-session
$ wkg fetch --config .wkg/config.toml
Error: failed to fetch dependencies for wit

Caused by:
0: failed to merge package from directory `wit`
1: interface not found in package
```

使用 `wkg fetch --config .wkg/config.toml` 重现所列案例，并追踪 `interface not found in package` 错误链的来源。Issue 未指定文件或测试；完成意味着让此次 fetch 失败更具体且更有用，然后验证命令的输出。

由索引模型根据 Issue 内容生成。