dsh-sandbox-micro: Moving DSH's Model Shell into a Linux Micro-VM

# Introduction If you are developing agents using DeepSeek Harness (DSH), the model's shell commands run on the host machine by default, and the isolation boundaries depend on the executor implementation. Windows also carries a legacy baggage: early approaches started npm `.CMD` shims via `cmd /c`, causing the `&`, quotes, and `%VAR%` within model commands to be parsed by the host shell first. To achieve "commands executing in a hard-isolated environment and argv not being wrapped by the host shell," a cleaner solution is needed. The `dsh-sandbox-micro` introduced below is the plugin bundle designed for this purpose: it replaces the model-visible `ctx.shell` with...