docs: clean up stale migration artifacts

Remove empty directories left by the test migration and replace the
"final cutover" language in docs/testing.md with standing PR guidance
that no longer references the completed migration.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

Author: morluto

docs/testing.md

@@ -109,6 +109,6 @@ for another.
 
 The PR acceptance target is a median `npm run check:pr` wall time below three
 minutes across three warm-build runs on the benchmark host. Keep Vitest caches
-cold unless separately identified. The final cutover also requires three
-consecutive deterministic passes without retries, `npm run verify:package`, and
-the applicable real-system lanes.
+cold unless separately identified. A PR that touches packaging or real-system
+behavior also requires `npm run verify:package` and the applicable
+`verify:*` lanes.