feat: add configurable vision focus prompts

backend/services/workflow_engine.py

@@ -23,6 +23,7 @@ class AgentStageConfig:
     tts_resource_id: str
     vision_enabled: bool
     vision_model_resource_id: str | None
+    vision_focus_prompt: str
     tool_ids: tuple[str, ...]
     state_variable_names: tuple[str, ...]
     knowledge_base_id: str | None
@@ -151,6 +152,7 @@ class WorkflowEngine:
             vision_model_resource_id=(
                 str(source.get("visionModelResourceId") or "") or None
             ),
+            vision_focus_prompt=str(source.get("visionFocusPrompt") or "").strip(),
             tool_ids=tuple(str(tool_id) for tool_id in source.get("toolIds") or []),
             # Variable write scope always belongs to the Agent node, even when
             # its selected tool resources come from Workflow defaults.