feat(FN-834): add per-workflow-step model overrides for execution

- Extend WorkflowStep type with optional modelProvider, modelId, validatorModelProvider, validatorModelId fields
- Add API endpoints (PATCH /api/workflow-steps/:id) to update model overrides on workflow steps
- Update executor to use per-step model overrides when running workflow step agents
- Add AgentDetailView support for displaying workflow step model configuration
- Add store and executor tests for new model override behavior

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

@@ -125,7 +125,9 @@ What the task should accomplish.
       "id": "WS-001",
       "name": "Documentation Review",
       "prompt": "Review the task changes...",
-      "enabled": true
+      "enabled": true,
+      "modelProvider": "anthropic",
+      "modelId": "claude-sonnet-4-5"
     }
   ]
 }