feat(soak): scaffold tests/soak package

Placeholder runner, tsconfig with @bot alias to tests/e2e/bot,
gitignored .env.stresstest + artifacts. Real behavior follows
in Task 10 onward.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/soak/.gitignore

@@ -0,0 +1,5 @@
+node_modules/
+dist/
+artifacts/
+.env.stresstest
+*.log