dsh-reveal-explorer: Open Current Workspace in the System File Manager

`dsh-reveal-explorer` is a plugin for DeepSeek Harness (DSH) designed to solve the hassle of manually locating paths and using `cd`. By adding an "Open in Explorer" button to the session header, the plugin invokes the system's default file manager with a single click to open the current workspace directory. Installation requires DSH version >= 0.1.1-rc.2 and a specified profile name, using the command `dsh plugin --profile <name> add @easytz/dsh-reveal-explorer`. The plugin supports Windows, macOS, and Linux, and automatically adapts to the corresponding system open command (e.g., `explorer.exe` or `open`).