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>

.storybook/main.ts

@@ -1,8 +1,18 @@
 import type { StorybookConfig } from '@storybook/angular';
+import remarkGfm from 'remark-gfm';
 
 const config: StorybookConfig = {
   stories: ['../src/**/*.mdx', '../src/**/*.stories.@(js|jsx|mjs|ts|tsx)'],
-  addons: ['@storybook/addon-a11y', '@storybook/addon-docs', '@storybook/addon-onboarding'],
+  addons: [
+    '@storybook/addon-a11y',
+    // remark-gfm so GFM pipe tables in *.mdx docs actually render (addon-docs
+    // doesn't parse them without it).
+    {
+      name: '@storybook/addon-docs',
+      options: { mdxPluginOptions: { mdxCompileOptions: { remarkPlugins: [remarkGfm] } } },
+    },
+    '@storybook/addon-onboarding',
+  ],
   framework: '@storybook/angular',
   // Serve the vendored CIBG package so preview-head.html can <link> its CSS (and its
   // relative font/icon/image url()s resolve) — mirrors index.html for the real app.