dsh-session-reference-copy: Copy Cross-Session References in DeepSeek Harness

`dsh-session-reference-copy` is a DeepSeek Harness (DSH) plugin designed to address the inconvenience of cross-session references. It adds a copy button to the header of the current session, writing a structured canonical mention (in JSON/UTF-8/base64url format) to the clipboard. By pasting this text, users can reference a snapshot of the current session in another session. The plugin supports readable labels, safe escaping, and failure feedback. Installation and uninstallation are performed via command-line operations, with changes taking effect after the browser is restarted.