dsh-multi-root: Mounting VS Code-style multi-root workspaces for DSH Web GUI

# Introduction When developing agents using the DSH Web GUI, a common frustration is that there is only one workspace: code resides in one directory, datasets in another, and references in a third. To enable the agent to operate across these directories, you must either manually move files into the same workspace or simply prevent the agent from accessing them. `dsh-multi-root` addresses this problem. Below is an introduction to the plugin: it adds a `Roots` (multi-root) panel to the sidebar, allowing an arbitrary number of independent folders to be mounted as workspace roots. Then, through a set of controlled host-side tools, it enables the agent to list/read/write/glob across these roots—drawing inspiration from VS Code.