dsh-subagent-watchdog: One-Time Automatic max-tokens Recovery for Native Resumable Sub-agents

The DSH plugin `dsh-subagent-watchdog` is designed to address the need to manually recover a subagent when it terminates due to insufficient `max-tokens`. It automatically performs one continuation via the official API, ensuring that each task is recovered at most once, with no loops or timers involved. Once installed, the plugin automatically continues the task when a termination is detected and stops intervening after completion; if it fails again, it notifies the parent agent and provides manual handling options. The plugin supports only specific versions of DSH and runs with the permissions of the current process.