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>

docs/TESTING-SUITES.md

@@ -72,6 +72,6 @@ Each matrix cell runs `unit`, `integration`, and `security` on every PR. `instal
 ## Best practices for forward-compat (Node 24/26)
 
 - Use `process.execPath` when spawning Node in tests so each matrix lane exercises the lane's Node version.
-- Avoid exact stack-trace or error-message prose assertions. Assert `err.code`, structured JSON, or a stable message substring instead — Node minor releases routinely tweak error wording.
+- Avoid stack-trace or error-message prose assertions. Assert `err.code`, structured JSON fields, or enums — Node minor releases routinely tweak error wording.
 - Prefer `node:test`, `node:assert/strict`, and `node:test` mocks. No external test frameworks.
 - Coverage uses `c8` and propagates `NODE_V8_COVERAGE` through the harness's child process.