dsh-proxy-env: DeepSeek Harness Proxy Environment Toggle Plugin

The `dsh-proxy-env` plugin aims to address the lack of flexibility in proxy management within DeepSeek Harness (DSH). It supports host-side global distribution and child-process environment injection, and provides a Web configuration card. Core features include `all` mode (proxies all traffic) and `include` mode (proxies only specified hosts). It supports proxy address configuration and a safe fault-tolerance mechanism, and provides a `/proxy` command-line interface as well as a Web interaction interface. After installation, the process must be restarted. It is compatible with DSH 0.1.2-rc.1 and above, and automatically restores the environment upon uninstallation.