dsh-deadman: Dead Man's Switch for DeepSeek Harness

`dsh-deadman` is a dead man's switch plugin for DeepSeek Harness (DSH), designed to address the issue of resources remaining unreleased in unattended long-running tasks when a process crashes or its quota is exhausted. It periodically monitors agent liveness and automatically executes a predefined cleanup command if no agent checks in. The plugin provides tool commands such as `deadman_arm` (arming) and `deadman_hold` (checking in), as well as the `/deadman` slash command, and supports installation and use on Web and desktop clients. Its core logic uses a grace window to determine whether to take action, includes a fail-open mechanism, and is suitable for scenarios such as overnight compilation and model training, but it must be installed in a long-running Profile.