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.

src/ClaudeDo.Ui/AppSettings.cs

@@ -16,6 +16,9 @@ public sealed class AppSettings
     public string DiffViewMode { get; set; } = "unified";
     public bool DiffWrapLines { get; set; }
 
+    /// <summary>Mission Control overview column count; 0 = auto (derived from the pane count).</summary>
+    public int MissionControlColumns { get; set; }
+
     private static readonly string DefaultConfigPath = Paths.Expand("~/.claudeDo/ui.config.json");
 
     /// Where this instance persists. Instance-level (not static) so tests can redirect it —