dsh-power-xc: DeepSeek Harness Power Management Plugin

`dsh-power-xc` is a power management plugin for DeepSeek Harness, designed to eliminate session interruptions caused by frequent restarts or shutdowns during development. The plugin provides page refresh, restart (with animation and configuration reload), and shutdown capabilities, and supports Web UI sidebar buttons, slash commands (e.g., `/restart`), and model tool calls. It runs as an independent engine with a safety confirmation mechanism and a 30-second timeout protection, and must be installed and used via the CLI on DSH ≥ 0.2.0.