feat(FN-883): add workflow step default-on support with task creation opt-out

- Add defaultOn field to WorkflowStep model, API endpoints (GET/POST/PATCH), and WorkflowStepManager UI
- Add skip-default-steps checkbox to TaskForm and NewTaskModal for explicit opt-out of default workflow steps
- Pre-select default workflow steps on new task creation forms automatically
- Add CSS styling for skip-default-steps toggle row
- Add comprehensive tests for WorkflowStepManager, TaskForm, and NewTaskModal components
- Add route tests for PATCH workflow-step endpoint and default selection logic
- Update README and task-structure reference docs

packages/cli/skill/fusion/references/task-structure.md

@@ -126,6 +126,7 @@ What the task should accomplish.
       "name": "Documentation Review",
       "prompt": "Review the task changes...",
       "enabled": true,
+      "defaultOn": true,
       "modelProvider": "anthropic",
       "modelId": "claude-sonnet-4-5"
     }