feat(content): add action costs and multi-yield schema

src/engine/__tests__/save.test.ts

@@ -15,7 +15,7 @@ function testContent() {
   return buildContent({
     resources: [{ id: 'gold', name: 'Gold', startAmount: 0 }],
     actions: [
-      { id: 'forage', name: 'Forage', durationMs: 3000, yields: { resourceId: 'gold', amount: 1 } },
+      { id: 'forage', name: 'Forage', durationMs: 3000, yields: [{ resourceId: 'gold', amount: 1 }] },
     ],
   });
 }