feat(cli): add Windows toast notification for task completion

[rekty]

{
"title": "feat(cli): add Windows toast notification for task completion",
"head": "rekty:feat/desktop-notification",
"base": "main",
"body": "Closes #1111\n\nWhen speakers are muted or the user steps away, the terminal bell (BEL character in the OSC title sequence) is easy to miss. This adds a native Windows toast notification that fires when a run completes so users always get a visual cue.\n\n### Changes\n\n- cli/src/utils/notification.ts (new) — notifyDesktop(title, body) uses .NET ToastNotificationManager via PowerShell. Works on every Windows 10+ machine without extra dependencies. Falls back gracefully on any error. notifyTaskComplete(agentName?) is the convenience wrapper.\n- cli/src/hooks/helpers/send-message.ts — calls notifyTaskComplete() after finalizeQueueState on successful completion.\n\n### Platform behavior\n\n| Platform | Notification |\n|---|---|\n| Windows | Native toast (via PowerShell .NET interop) |\n| macOS / Linux | No-op (terminal bell already provides the cue) |\n\nThe Desktop (Electron) app can also import notifyTaskComplete directly for its own completion events.\n\n### Verification\n\n- bun test cli/src/hooks/helpers/__tests__/send-message.test.ts — pre-existing failure (missing web-tree-sitter dep), unrelated to this change\n- notification.ts is self-contained with no external dependencies"
}