feat: scaffold Astro project replacing Gatsby

- Remove gatsby-config.js, yarn.lock, src/gatsby-theme-blog/
- Create package.json with astro, @astrojs/mdx, @astrojs/rss deps
- Create astro.config.mjs with site/base for GitHub Pages
- Create tsconfig.json extending astro/tsconfigs/strict
- Create src/env.d.ts with astro/client reference
- Create minimal src/pages/index.astro placeholder
- Update .gitignore: replace gatsby section with astro (dist/, .astro/)
- Add scaffold verification tests
- Verified: npm run build succeeds with output in dist/

astro.config.mjs

@@ -0,0 +1,10 @@
+import { defineConfig } from 'astro/config';
+import mdx from '@astrojs/mdx';
+
+export default defineConfig({
+  site: 'https://kenotron.github.io',
+  base: '/crashtestdev',
+  integrations: [
+    mdx(),
+  ],
+});