dsh-notifier: Cross-window Task Reminder Plugin for DSH

dsh-notifier is a DSH task notification plugin that brings the Hook notification experience of Codex and Claude Code to DSH, addressing the issue of missed task state changes when users switch windows. It monitors four event categories: task completion, permission approval, user inquiries, and task errors. Notifications are delivered via three channels: in-page toasts, system notification center banners, and cross-window floating panels. Popups for decision-making events (approval, inquiries) remain pinned until resolved, while popups for task completion and errors disappear automatically after 10 seconds. On macOS, it utilizes a built-in Swift notifier to render floating panels that float above all windows and fullscreen applications; Windows and Linux use PowerShell and zenity, respectively, but have not been verified on real hardware. v0.4