feat(tui): OSC 9 desktop notifications for toasts (#4454)

[mcheviron]

Summary

Implements OSC 9 desktop notifications for important TUI events, as proposed in #4454.

• Add a small OSC 9 helper for emitting terminal notifications from the TUI.
• Wire desktop notifications through the existing toast system so any toast.show(...) can surface a desktop notification.
• Gate notifications behind config.tui.notifications.desktop (defaulting to enabled when unset).

Implementation Details

OSC 9 helper

• New file packages/opencode/src/cli/cmd/tui/util/osc.ts exposes:
  • osc9(text: string) which writes ESC ] 9;{text} BEL to stdout, wrapped for tmux when $TMUX is set.
• This keeps OSC 9 usage encapsulated and aligned with how Codex/Claude use terminal notifications.

Toast integration

• packages/opencode/src/cli/cmd/tui/ui/toast.tsx:
  • ToastProvider now reads sync.data.config.tui.notifications.desktop via useSync.
  • toast.show(...) calls sendDesktopNotification(summary) once per toast (using the toast title or a default "Notification").
  • If config.tui.notifications.desktop === false, desktop notifications are suppressed but visual toasts still render.
• This keeps OSC 9 as a purely TUI concern and avoids sprinkling notification calls around the codebase.

Session toasts as first OSC 9 triggers

• packages/opencode/src/cli/cmd/tui/routes/session/index.tsx:
  • Track status and permissions per route.sessionID and detect transitions:
    • non-idle -> idle → show "Agent finished / Session is now idle" toast.
    • permissions.length: 0 -> >0 → show "Permission required / Review and approve requested actions" toast.
  • Because these go through toast.show(...), they also trigger OSC 9 notifications (when enabled), matching the behavior described in #4454.

[mcheviron]

@rekram1-node do you think this could be added soon? I am not sure if it's the type of feature that needs architectural stuff or not