feat(worker): expose maxParallelExecutions in get_app_settings

The list-handler prompt (MergeHelperDefault) tells the handler to read
maxParallelExecutions via get_app_settings, but the DTO never carried
the field. Handler had to fall back to reading app_settings directly
from the DB on the 2026-07-29 E2E run.

tests/ClaudeDo.Worker.Tests/External/AppSettingsMcpToolsTests.cs

@@ -0,0 +1,31 @@
+using ClaudeDo.Data.Models;
+using ClaudeDo.Worker.External;
+using ClaudeDo.Worker.Tests.Infrastructure;
+using Microsoft.EntityFrameworkCore;
+
+namespace ClaudeDo.Worker.Tests.External;
+
+public sealed class AppSettingsMcpToolsTests : IDisposable
+{
+    private readonly DbFixture _db = new();
+
+    public void Dispose() => _db.Dispose();
+
+    [Fact]
+    public async Task GetAppSettings_ReturnsMaxParallelExecutionsFromDb()
+    {
+        using (var ctx = _db.CreateContext())
+        {
+            var row = await ctx.AppSettings.FirstOrDefaultAsync(s => s.Id == AppSettingsEntity.SingletonId)
+                ?? throw new InvalidOperationException("Expected seeded app_settings row.");
+            row.MaxParallelExecutions = 3;
+            await ctx.SaveChangesAsync();
+        }
+
+        var sut = new AppSettingsMcpTools(_db.CreateFactory());
+
+        var result = await sut.GetAppSettings(CancellationToken.None);
+
+        Assert.Equal(3, result.MaxParallelExecutions);
+    }
+}