dsh-hybrid-notify: DSH Multi-Channel Notification Plugin

The `dsh-hybrid-notify` plugin is designed specifically for DSH agents to address missed messages during long-running sessions. It monitors window visibility to intelligently switch between in-page toasts, PWA system notifications, and browser notifications, and uses the Web Audio API to synthesize audio alerts for critical events. The plugin supports event-level and channel-level toggle controls, features configuration persistence, zero network access, and compliance with browser autoplay policies, ensuring that background task information is accurately captured.