fix(3597): resolve CR threads — drop substring-assertion guidance, use $testDir

Two CodeRabbit threads from PR #3649 review:

- docs/TESTING-SUITES.md:75 — removed the "stable message substring"
  fallback from the error-assertion guidance. Project rule (per the
  no-source-grep lint and lint-no-source-grep.cjs) is structured/typed
  checks only — err.code, JSON fields, enums. Substring matching
  re-introduces the exact prose-coupling we banned.

- scripts/run-tests.cjs:106 — the "no test files found" error now
  reports the resolved testDir variable instead of the hardcoded
  'tests/' string, so when GSD_TEST_DIR points elsewhere the message
  names the actual directory the harness searched.

Local: docker gsd-test-summary 11224/0 on holodeck.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scripts/run-tests.cjs

@@ -103,7 +103,7 @@ function main() {
     .sort();
 
   if (allFiles.length === 0) {
-    console.error('No test files found in tests/');
+    console.error(`No test files found in ${testDir}`);
     process.exit(1);
   }