dsh-fetch-file: DSH Plugin to Download Any URL as a Workspace File

`dsh-fetch-file` is a DeepSeek Harness (DSH) plugin that fills the gap left by `web_fetch`, which cannot handle binary files. The plugin supports downloading any URL as a workspace file via streaming writes, with a 200MB size limit and path safety fencing. It has no dependencies and is pure ESM; after installation, simply call the `download` tool to use it. It supports automatic redirect following and returns download results using normalized JSON values.