dsh-session-prompt: Inject Custom Content at the Top of the System Prompt

The DeepSeek Harness plugin `dsh-session-prompt` addresses the pain point of configuring preamble instructions for global sessions in agent development. It allows users to edit and inject custom prompts in real time through the Web UI, applying them at the top of the system prompt without modifying source code or restarting the service. Installation requires DSH version ≥ `0.1.7-rc.1`. Note that the injection scope covers all sessions globally, and the plugin will reject saving text containing a complete but unclosed `{{…}}` template group to prevent request errors.