Delayed wake-up for DeepSeek Harness

Key features

• The timer lives in the plugin apply scope, not ctx.jobs (which dies with the owning turn).
• setTimeout().unref() lets the process exit normally when only the timer is left — it never blocks the DSH lifecycle.
• agent.followup() is the runtime's official wake channel: a wake submitted while the agent is idle always opens a new turn boundary (see the runtime-types comments), so cross-turn waking is a supported semantic.