feat(worker): configurable max parallel task executions

Add a "Max parallel executions" setting to the General settings tab so
the queue can run more than one task concurrently. QueueService now
tracks multiple active slots and reads the limit from app settings each
cycle, so changes take effect without restarting the worker.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/ClaudeDo.Ui/Services/WorkerClient.cs

@@ -450,6 +450,7 @@ public sealed record AppSettingsDto(
     string DefaultModel,
     int DefaultMaxTurns,
     string DefaultPermissionMode,
+    int MaxParallelExecutions,
     string WorktreeStrategy,
     string? CentralWorktreeRoot,
     bool WorktreeAutoCleanupEnabled,

src/ClaudeDo.Ui/ViewModels/Modals/Settings/GeneralSettingsTabViewModel.cs

@@ -9,6 +9,7 @@ public sealed partial class GeneralSettingsTabViewModel : ViewModelBase
     [ObservableProperty] private string _defaultModel = ModelRegistry.DefaultAlias;
     [ObservableProperty] private int _defaultMaxTurns = 100;
     [ObservableProperty] private string _defaultPermissionMode = PermissionModeRegistry.DefaultMode;
+    [ObservableProperty] private int _maxParallelExecutions = 1;
 
     public IReadOnlyList<string> Models { get; } = ModelRegistry.Aliases;
     public IReadOnlyList<string> PermissionModes { get; } = PermissionModeRegistry.Modes;
@@ -17,6 +18,8 @@ public sealed partial class GeneralSettingsTabViewModel : ViewModelBase
     {
         if (DefaultMaxTurns < 1 || DefaultMaxTurns > 200)
             return "Max turns must be between 1 and 200.";
+        if (MaxParallelExecutions < 1 || MaxParallelExecutions > 20)
+            return "Max parallel executions must be between 1 and 20.";
         return null;
     }
 }

src/ClaudeDo.Ui/ViewModels/Modals/SettingsModalViewModel.cs

@@ -42,6 +42,7 @@ public sealed partial class SettingsModalViewModel : ViewModelBase
                 General.DefaultModel = dto.DefaultModel ?? "sonnet";
                 General.DefaultMaxTurns = dto.DefaultMaxTurns;
                 General.DefaultPermissionMode = dto.DefaultPermissionMode ?? "auto";
+                General.MaxParallelExecutions = dto.MaxParallelExecutions;
                 Worktrees.WorktreeStrategy = dto.WorktreeStrategy ?? "sibling";
                 Worktrees.CentralWorktreeRoot = dto.CentralWorktreeRoot;
                 Worktrees.WorktreeAutoCleanupEnabled = dto.WorktreeAutoCleanupEnabled;
@@ -69,6 +70,7 @@ public sealed partial class SettingsModalViewModel : ViewModelBase
                 General.DefaultModel ?? "sonnet",
                 General.DefaultMaxTurns,
                 General.DefaultPermissionMode ?? "auto",
+                General.MaxParallelExecutions,
                 Worktrees.WorktreeStrategy ?? "sibling",
                 string.IsNullOrWhiteSpace(Worktrees.CentralWorktreeRoot) ? null : Worktrees.CentralWorktreeRoot,
                 Worktrees.WorktreeAutoCleanupEnabled,

src/ClaudeDo.Ui/Views/Modals/SettingsModalView.axaml

@@ -75,6 +75,14 @@
                             HorizontalAlignment="Stretch"/>
                 </StackPanel>
               </Grid>
+              <StackPanel Spacing="4">
+                <TextBlock Classes="field-label" Text="Max parallel executions"/>
+                <NumericUpDown Value="{Binding General.MaxParallelExecutions, Mode=TwoWay}"
+                               Minimum="1" Maximum="20" Increment="1" FormatString="0"
+                               HorizontalAlignment="Left" Width="140"/>
+                <TextBlock Text="How many queued tasks the worker runs at once."
+                           Opacity="0.6" FontSize="12"/>
+              </StackPanel>
             </StackPanel>
           </ScrollViewer>
         </TabItem>