dsh-battery-life: Battery Runtime Estimation

The `dsh-battery-life` plugin for DeepSeek Harness (DSH) is implemented in pure Node.js and enables clients to retrieve device battery status and battery-life estimates. The tool runs entirely locally without network access and supports parameterized calculations. Developers can install it via a command and use its registered `battery_life` tool to integrate system hardware status query capabilities. The plugin is open source and suitable for running in a DSH environment; please review the source code and license before use.