chore(claude-do): Make add_task optional params actually optional

add_task currently marks description, createdBy, and queueImmediately as required, forcing callers to invent values for fields that have obvious defaults.

Fix: make them optional with sensible defaults — description: null, queueImmediately: false, createdBy: server default like "mcp". Keep only listId and title as truly required.

ClaudeDo-Task: b9fadf0b-a20e-4deb-932d-29ef9c0b83f3

src/ClaudeDo.Worker/External/ExternalMcpService.cs

@@ -90,17 +90,15 @@ public sealed class ExternalMcpService
     public async Task<TaskDto> AddTask(
         string listId,
         string title,
-        string? description,
-        string createdBy,
-        bool queueImmediately,
-        CancellationToken cancellationToken)
+        string? description = null,
+        string? createdBy = null,
+        bool queueImmediately = false,
+        CancellationToken cancellationToken = default)
     {
         if (string.IsNullOrWhiteSpace(listId))
             throw new InvalidOperationException("listId is required.");
         if (string.IsNullOrWhiteSpace(title))
             throw new InvalidOperationException("title is required.");
-        if (string.IsNullOrWhiteSpace(createdBy))
-            throw new InvalidOperationException("createdBy is required.");
 
         var list = await _lists.GetByIdAsync(listId, cancellationToken)
             ?? throw new InvalidOperationException($"List {listId} not found.");
@@ -114,7 +112,7 @@ public sealed class ExternalMcpService
             Status = TaskStatus.Idle,
             CreatedAt = DateTime.UtcNow,
             CommitType = list.DefaultCommitType,
-            CreatedBy = createdBy,
+            CreatedBy = createdBy.NullIfBlank() ?? "mcp",
         };
         await _tasks.AddAsync(entity, cancellationToken);