feat(a11y): WP-01 — axe-on-every-story CI gate

Turn the interactive Storybook a11y addon into a build gate:
- @storybook/test-runner + axe-playwright over the static build
  (.storybook/test-runner.ts reads the a11y tags from story context)
- test-storybook / test-storybook:ci scripts; storybook-a11y CI job
- triage: escape-hatch a11y.disable on stories whose display:contents
  wrapper splits <ul>/<li> or <dl>/<dt>/<dd> (structural, deferred to
  WP-11/WP-12, each with justification + cross-ref)
- fix trivial violations: footer/wizard-shell contrast, text-input label,
  wizard stories missing provideApiClient

Verified: broken story fails the gate; 133 stories pass.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.storybook/test-runner.ts

@@ -0,0 +1,21 @@
+import type { TestRunnerConfig } from '@storybook/test-runner';
+import { getStoryContext } from '@storybook/test-runner';
+import { injectAxe, checkA11y } from 'axe-playwright';
+
+const config: TestRunnerConfig = {
+  async preVisit(page) {
+    await injectAxe(page);
+  },
+  async postVisit(page, context) {
+    const storyContext = await getStoryContext(page, context);
+    if (storyContext.parameters?.a11y?.disable) return;
+
+    await checkA11y(page, '#storybook-root', {
+      axeOptions: storyContext.parameters?.a11y?.config,
+      detailedReport: true,
+      detailedReportOptions: { html: true },
+    });
+  },
+};
+
+export default config;