dsh-timer-scheduler: Let the agent automatically wake up later to check background tasks

`dsh-timer-scheduler` is an MIT-licensed DSH plugin designed to address the issue where manual prompting is required to check after an Agent starts a background task. The plugin provides a one-time reminder tool for Agents, supporting the setting of reminders via relative or absolute time, and automatically waking up the Agent to continue action when the reminder expires, without manual intervention. Core functions include `schedule_reminder` (supports associating background tasks, automatically cancelling the reminder when the task is completed), `list_reminders`, and `cancel_reminder`. Reminder data is persisted and remains valid after a restart, while the Web UI top bar displays reminders and countdowns in real-time. Installation requires mounting from source to the Web profile and restarting the service; if using specific tools to filter...