dsh-clipfile: Mounting Files and Folders in DSH Composer

The `dsh-clipfile` plugin adds native file mounting capability to the DeepSeek Harness (DSH) Web editor, addressing the bottleneck in file interactions. It supports clipboard pasting, a native file picker, fuzzy path search (@mentions), and paginated Markdown conversion. Attachments are displayed as tags and are scoped to the current session. The plugin relies on the macOS environment for a zero-copy mechanism and can be installed and used via `dsh plugin add`.