Harness: print non-string check details as JSON

check() appended the detail by string concatenation, so an object
argument showed up as "[object Object]" in the log (27-console.mjs).

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

tools/lib/harness.mjs

@@ -382,7 +382,8 @@ export async function runChecks(options, body) {
   let failures = 0;
   let checkCount = 0;
   function check(name, ok, detail = '') {
-    console.log(`${ok ? 'PASS' : 'FAIL'}  ${name}${detail ? ' – ' + detail : ''}`);
+    const text = typeof detail === 'string' ? detail : JSON.stringify(detail);
+    console.log(`${ok ? 'PASS' : 'FAIL'}  ${name}${text ? ' – ' + text : ''}`);
     checkCount++;
     if (!ok) failures++;
   }