feat: add RuntimeStep union type for procedural custom steps

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

frontend/src/types/tree.ts

@@ -123,6 +123,18 @@ export interface ProceduralStep {
   reference_url?: string
 }
 
+export interface CustomProceduralStep {
+  id: string
+  type: 'procedure_step'
+  title: string
+  description?: string
+  content_type: 'action'
+  commands?: CommandBlock[]
+  isCustom: true
+}
+
+export type RuntimeStep = ProceduralStep | CustomProceduralStep
+
 export interface ProceduralTreeStructure {
   steps: ProceduralStep[]
 }