feat(storybook): wire @storybook/test-runner for story smoke tests

Every story is now executed as a smoke test (mount + no console errors)
via @storybook/test-runner. New script: pnpm test:stories runs
build-storybook then test-storybook against the static build.

Spec: §6.8

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

package.json

@@ -11,6 +11,7 @@
     "lint": "turbo run lint",
     "test": "turbo run test",
     "test:e2e": "turbo run test:e2e",
+    "test:stories": "turbo run test:stories",
     "typecheck": "turbo run typecheck",
     "format": "prettier --write \"**/*.{ts,tsx,js,jsx,json,md}\"",
     "format:check": "prettier --check \"**/*.{ts,tsx,js,jsx,json,md}\""