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.

src/ClaudeDo.Worker/External/AppSettingsMcpTools.cs

@@ -8,6 +8,7 @@ namespace ClaudeDo.Worker.External;
 
 public sealed record AppSettingsReadDto(
     string DefaultModel, int DefaultMaxTurns, string DefaultPermissionMode,
+    int MaxParallelExecutions,
     string WorktreeStrategy, string? CentralWorktreeRoot,
     bool WorktreeAutoCleanupEnabled, int WorktreeAutoCleanupDays);
 
@@ -18,13 +19,14 @@ public sealed class AppSettingsMcpTools
 
     public AppSettingsMcpTools(IDbContextFactory<ClaudeDoDbContext> dbFactory) => _dbFactory = dbFactory;
 
-    [McpServerTool, Description("Read the worker's app-level defaults (model, max turns, permission mode, worktree strategy). Read-only.")]
+    [McpServerTool, Description("Read the worker's app-level defaults (model, max turns, permission mode, max parallel execution slots, worktree strategy). Read-only.")]
     public async Task<AppSettingsReadDto> GetAppSettings(CancellationToken cancellationToken)
     {
         using var ctx = await _dbFactory.CreateDbContextAsync(cancellationToken);
         var row = await new AppSettingsRepository(ctx).GetAsync(cancellationToken);
         return new AppSettingsReadDto(
             row.DefaultModel, row.DefaultMaxTurns, row.DefaultPermissionMode,
+            row.MaxParallelExecutions,
             row.WorktreeStrategy, row.CentralWorktreeRoot,
             row.WorktreeAutoCleanupEnabled, row.WorktreeAutoCleanupDays);
     }