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>

docs/backlog/WP-01-axe-ci-gate.md

@@ -1,6 +1,6 @@
 # WP-01 — Axe-on-every-story CI gate
 
-Status: todo
+Status: done (pending commit)
 Phase: 0 — enforcement & gates
 
 ## Why
@@ -59,11 +59,11 @@ build", so every story added or changed by later WPs is automatically covered.
 
 ## Acceptance criteria
 
-- [ ] `npm run test-storybook:ci` passes locally against a fresh `storybook-static/`.
-- [ ] CI has a `storybook-a11y` job; an axe violation makes it fail (verify once with a
-      deliberately broken story, then revert).
-- [ ] Every `a11y.disable` skip carries a justification comment + WP cross-ref.
-- [ ] Backlog README GREEN updated.
+- [x] `npm run test-storybook:ci` passes locally against a fresh `storybook-static/`.
+- [x] CI has a `storybook-a11y` job; an axe violation makes it fail (verified once with a
+      deliberately broken story, then reverted).
+- [x] Every `a11y.disable` skip carries a justification comment + WP cross-ref.
+- [x] Backlog README GREEN updated.
 
 ## Verification