docs(storybook): render MDX tables + add i18n & testing-strategy docs/skill

- fix: wire remark-gfm into addon-docs so GFM pipe tables in *.mdx render
  (previously raw text in cibg-gaps/layers/atomic-design docs)
- add src/docs/i18n.mdx (Foundations/Internationalization): the $localize
  locale seam + how to test languages without coupling to copy
- add src/docs/testing.mdx (Foundations/Testing strategy): per-layer spec
  matrix, house style, Storybook a11y gate, GREEN gate
- add .claude/skills/test-strategy skill

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

package.json

@@ -55,6 +55,7 @@
     "jsdom": "^29.0.0",
     "nswag": "^14.7.1",
     "prettier": "^3.8.1",
+    "remark-gfm": "^4.0.1",
     "storybook": "^10.4.6",
     "typescript": "~6.0.2",
     "typescript-eslint": "^8.62.0",