dsh-extension-workbook: High-Fidelity Excel Workbook Inspection and Editing Plugin

`dsh-extension-workbook` is a plugin for DeepSeek Harness (DSH) designed to handle Excel files securely and with high fidelity. Ported from an MIT-licensed project, it provides inspection, rendering, comparison, and validation capabilities. A core feature is its "fail-closed" strategy: it refuses to process complex formats such as encrypted or ZIP64 files and never executes macro code. The plugin ensures the safety and auditability of editing operations through dry-run planning, SHA-256 verification, and an atomic write process, making it suitable for automation scenarios with stringent data-integrity requirements.