dsh-approval-explain: Add an Explanation Button to the DeepSeek Harness Approval Card

The dsh-approval-explain plugin is designed to address an issue in DeepSeek Harness (DSH) approval cards where the details area remains empty for non-`command` tools such as `write` and `edit`. By adding an “Explain” button or using the `/explain` command, the plugin leverages an LLM to generate a three-line summary including the operation content, affected files, and a security assessment. It supports three parameter shapes: `command`, `file_path`, and `path`; visually reuses the native button style; and is non-invasive. Installation requires DSH 0.1.5-rc.1 or later, and it currently supports only the Web GUI. Using it incurs LLM call costs, is recorded in logs, and does not affect the actual approval process.