feat(mission-control): column picker for the overview grid

Auto keeps the pane-count heuristic; 1-4 fixes the column count (capped at
the pane count). Persisted to ui.config.json as MissionControlColumns.

tests/ClaudeDo.Ui.Tests/ViewModels/MissionControlViewModelTests.cs

@@ -491,6 +491,33 @@ public class MissionControlViewModelTests : IDisposable
         Assert.Equal(2, vm.ColumnCount);
     }
 
+    [Fact]
+    public async Task SetColumns_OverridesAutoCapsAtPaneCountAndPersists()
+    {
+        var path = Path.Combine(Path.GetTempPath(), $"claudedo_mc_cfg_{Guid.NewGuid():N}.json");
+        try
+        {
+            var worker = new FakeWorker();
+            using var vm = new MissionControlViewModel(new TestDbFactory(NewContext), worker,
+                new UsagePillViewModel(worker), new AppSettings { ConfigPath = path });
+            for (var i = 1; i <= 4; i++) await vm.OpenConPtySessionAsync($"t{i}");
+            Assert.Equal(2, vm.ColumnCount); // auto: 4 panes -> 2x2
+
+            vm.SetColumnsCommand.Execute("3");
+            Assert.Equal(3, vm.ColumnCount);
+
+            vm.SetColumnsCommand.Execute("4");
+            vm.Panes[0].CloseCommand.Execute(null);
+            Assert.Equal(3, vm.ColumnCount); // never more columns than panes
+
+            Assert.Equal(4, AppSettings.Load(path).MissionControlColumns);
+
+            vm.SetColumnsCommand.Execute("0");
+            Assert.Equal(2, vm.ColumnCount);
+        }
+        finally { try { File.Delete(path); } catch { } }
+    }
+
     [Fact]
     public async Task CloseConPtySession_RemovesFromConPtySessionsAndPanes()
     {