dsh-backup-retention: Backup Capacity Planning

dsh-backup-retention is a pure Node.js plugin for DeepSeek Harness (DSH) that locally calculates backup capacity planning and retention policies without network dependencies. It supports precise parameter calculations and returns detailed results. It requires the dependencies `@deepseek-ai/cordis` and `@deepseek-ai/dsh-tools`. The installation command is `dsh plugin add github:uckkk/dsh-backup-retention`, making it suitable for quickly estimating backup scenarios locally.