dsh-schedule-command: In-Session Scheduled Task Management Plugin

`dsh-schedule-command` is a DeepSeek Harness plugin that allows users to create and manage local scheduled tasks in conversations via the `/schedule` command. Maintained by user piaohua, the plugin supports defining one-time or recurring execution plans in natural language and automatically mounts its runtime upon installation. Users can view and delete tasks, but tasks only trigger while the session is active; after sleep recovery, only the latest missed run is replayed. In addition, the underlying v1 implementation does not support Cron, so complex calendar rules fall back to fixed intervals.