feat(custom-fields) : add machineContextOnly toggle in structure editors

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/app/composables/usePieceStructureEditorLogic.ts

@@ -88,6 +88,7 @@ const toEditorField = (
     ...(typeof input?.id === 'string' && input.id ? { id: input.id } : {}),
     ...(typeof input?.customFieldId === 'string' && input.customFieldId ? { customFieldId: input.customFieldId } : {}),
     orderIndex: typeof input?.orderIndex === 'number' ? input.orderIndex : index,
+    machineContextOnly: Boolean(input?.machineContextOnly),
   }
 }
 
@@ -162,6 +163,7 @@ const buildPayload = (
         type,
         required,
         orderIndex: index,
+        machineContextOnly: Boolean(field.machineContextOnly),
       }
 
       if (field.id) {
@@ -286,6 +288,7 @@ export function usePieceStructureEditorLogic(deps: Deps) {
     type: 'text',
     required: false,
     optionsText: '',
+    machineContextOnly: false,
     orderIndex,
   })