feat(worker): default to claude-sonnet-4-6 when no model configured

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/ClaudeDo.Worker/Runner/TaskRunner.cs

@@ -80,7 +80,7 @@ public sealed class TaskRunner
             // Resolve config: task overrides > list config > null.
             var listConfig = await _listRepo.GetConfigAsync(task.ListId, ct);
             var resolvedConfig = new ClaudeRunConfig(
-                Model: task.Model ?? listConfig?.Model,
+                Model: task.Model ?? listConfig?.Model ?? "claude-sonnet-4-6",
                 SystemPrompt: task.SystemPrompt ?? listConfig?.SystemPrompt,
                 AgentPath: task.AgentPath ?? listConfig?.AgentPath,
                 ResumeSessionId: null