dsh-multi-workspace: DSH Multi-Workspace File Writing Plugin

`dsh-multi-workspace` is a plugin for DeepSeek Harness (DSH) that addresses the limitation of the default sandbox, which allows writing only to a single working directory. By extending the sandbox policy and wrapping filesystem tools, it automatically grants write access to all registered workspaces. Once installed, users can perform file operations across multiple workspaces directly, without additional configuration. However, note that underlying Shell commands remain limited to a single workspace, and the silent retry mechanism on write failures requires workspaces to be trusted.