dsh-polling: Add polling and scheduled task capabilities to DeepSeek Harness

dsh-polling is an MIT-licensed plugin for DeepSeek Harness designed to transform naturally described recurring tasks into Cron-driven scheduled polling tasks. It supports creating, editing, and managing tasks via natural language, making it suitable for scenarios such as periodically checking folders and web pages, scraping data, generating daily and weekly reports, and executing scripts. Core features include automatically parsing task requirements described in natural language using built-in model tools; executing tasks in independent real sessions without interrupting the current conversation, supporting multiple execution modes; and providing a Web UI for viewing task lists, searching, immediate execution, starting, stopping, and deleting tasks. The scheduling mechanism features a smart catch-up function (executing the most recent missed task upon startup after missing a scheduled time) and an anti-overlapping mechanism (newly triggered tasks during execution are skipped). Installation requires the dsh CLI.