dsh-file-menu: Adding a Codex-Style Context Menu to the DeepSeek Harness Web UI

The `dsh-file-menu` plugin adds right-click menu functionality missing from the native DeepSeek Harness (DSH) Web client. It provides a Codex-style context menu based on cursor position, supporting text copy/paste, copying file paths, showing files in a file manager, and opening files with their default application. The plugin implements file manager interactions via Electron main-process routing or system commands and is released under the MIT License.