dsh-wsl-fetch: Proxy-Aware web_fetch in WSL

`dsh-wsl-fetch` is a tool that resolves the issue where DeepSeek Harness's `web_fetch` bypasses the proxy in WSL environments. It registers a `wsl-proxy` provider and uses `ProxyAgent` to take over requests and forward them through the Windows HTTP proxy. The plugin is enabled via the `HTTP(S)_PROXY` environment variables, supports custom timeout, redirect, and retry policies, but cannot handle local URLs or Cloudflare WAF blocks. After installation, the model ID must be manually configured in `settings.yaml`.