dsh-plugin-heartbeat: Add a scheduled heartbeat to the DeepSeek Harness agent

`dsh-plugin-heartbeat` is an MIT-licensed plugin for DeepSeek Harness (DSH), designed to address the issue where Agent progress is invisible due to passive waiting for user instructions. By monitoring the `agent/created` event, mounting an independent timer for each root Agent, and utilizing `agent.followup()` to inject synthetic messages, the plugin prompts the Agent to proactively report progress, risks, or status when idle. Core features include support for busy-idle adaptive scheduling and message queue management, a backoff mechanism (with gradually increasing intervals) and hard stop logic (stopping after long periods without human interaction and reviving upon receiving user messages), pre-delivery context compression to reduce load, as well as hourly frequency control and foldable display optimization. Installation requires using `dsh`.