[Repo Assist] fix: respect sound=false in system.notify node notifications

[github-actions]

🤖 This is an automated pull request from Repo Assist.

Summary

When an agent invokes system.notify with sound=false, the PlaySound field in SystemNotifyArgs was correctly populated by SystemCapability — but silently ignored in OnNodeNotificationRequested. The toast was always shown with sound regardless of the per-notification flag.

Root Cause

SystemCapability.HandleNotifyAsync correctly reads the sound argument:

var sound = GetBoolArg(request.Args, "sound", true);
// ...
NotifyRequested?.Invoke(this, new SystemNotifyArgs { PlaySound = sound, ... });

But OnNodeNotificationRequested in App.xaml.cs never read args.PlaySound:

new ToastContentBuilder()
    .AddText(args.Title)
    .AddText(args.Body)
    .Show();  // ← PlaySound completely ignored

The existing test (CapabilityTests.Notify_RaisesEvent_WithArgs) already verifies that PlaySound=false is propagated to the event args — the gap is the UI handler.

Fix

var builder = new ToastContentBuilder()
    .AddText(args.Title)
    .AddText(args.Body);

if (!args.PlaySound)
    builder.AddAudio(silent: true);

builder.Show();

Relationship to PR #95

PR #95 applies the user's global NotificationSound preference to all toasts. This PR handles a different concern: respecting the per-notification sound=false flag sent by the agent. Both fixes are complementary and non-conflicting.

Test Status

• ✅ All 503 shared tests pass, 18 skipped (Windows-only infrastructure): dotnet test tests/OpenClaw.Shared.Tests/
• ⚠️ WinUI app cannot be built on the Linux CI runner (NETSDK1100), so App.xaml.cs changes are verified by inspection. The logic is a straightforward null-guard on a boolean field already confirmed by the existing capability test.

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[shanselman]

Superseded by PR #125 which applied NotificationSound to all toast call sites including system.notify.