dsh-clipboard: Zero-Dependency Writes to the System Clipboard

`dsh-clipboard` is a zero-dependency DSH plugin that allows an agent to write content directly to the system clipboard. It supports macOS, Windows, and Linux, returns standardized JSON data, and includes automatic self-check capabilities. The plugin is designed to simplify agent workflows through a “handoff output” step, eliminating the need for users to manually copy and paste. Installation relies on system-provided commands (such as `pbcopy`, `clip`, and `xclip`), and Linux requires an X11 or Wayland session.