dsh-subagent-watchdog: Reporting the death of subagents to the parent agent

DSH's AgentStatus only includes 'idle' and 'running'; it does not include 'failed'. When a child agent dies due to upstream disconnection, the parent agent still displays 'idle', making it indistinguishable from a normal completion or a session waiting state. Consequently, it can only be determined by polling the task board. Lequait/dsh-subagent-watchdog monitors 'agent/error' in the root scope. After confirming that the child agent has not recovered, it uses `parent.followup` to deliver a processable interrupt notification to the parent agent. It does not retry, nor does it modify session history; its sole responsibility is to report the failure. This paper presents three installation methods, configuration options, and known limitations.