dsh-workspace-open: Open Folders Directly in the DSH Workspace List

The `dsh-workspace-open` plugin adds a one-click workspace folder opening feature to DeepSeek Harness (DSH). Clicking the button in the left-side Workspace list opens the corresponding folder in the system's default file manager, supporting Windows, macOS, and Linux. The installation command is `dsh plugin --profile web add ./dsh-workspace-open`. The plugin uses a dual half-sector architecture, preferentially attempting to use a future slot and automatically falling back to DOM adaptation mode, applying only to actual workspace rows.