test: harden decision storage round-trip evidence

plugins/reporting-governance/src/storage/decision-store.mjs

@@ -39,5 +39,12 @@ export function createFileDecisionStore({ decisionsDir, repoRootOverride } = {})
         artifactPath: resolvedPath,
       };
     },
+    consume(artifactPath, consumer) {
+      if (typeof consumer !== 'function') {
+        throw new Error('decision store consumer must be a function');
+      }
+      const loaded = this.load(artifactPath);
+      return consumer(loaded);
+    },
   };
 }