Tests: point the skill-behavior harness at the launcher and engine binary

The bash tool exports IMPECCABLE_BIN so the staged skill's launcher runs
without a download; scenarios assert on 'impeccable context' instead of
context.mjs and skip without a binary.

Prepared with AI assistance (Claude Code).

tests/skill-behavior/workflow-contract.test.mjs

@@ -13,6 +13,8 @@ import {
   runTurn,
   fileLoaded,
   summarizeTrace,
+  ENGINE_BIN,
+  ENGINE_MISSING_MESSAGE,
 } from './harness.mjs';
 import { detectProvider, getModel, hasKey, resolveModelList, PROVIDERS } from './providers.mjs';
 import { PRODUCT_MD_SAMPLE, DESIGN_MD_SAMPLE } from './fixtures.mjs';
@@ -107,6 +109,10 @@ for (const modelId of resolveModelList()) {
       it(`skipped — ${PROVIDERS[provider].envKey} is unset`, { skip: true }, () => {});
       return;
     }
+    if (!ENGINE_BIN) {
+      it(`skipped — ${ENGINE_MISSING_MESSAGE}`, { skip: true }, () => {});
+      return;
+    }
     const model = getModel(modelId);
 
     it('fresh init asks and writes PRODUCT without inventing a visual system', async () => {