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/

package.json

@@ -2,17 +2,19 @@
   "name": "crashtestdev",
   "private": true,
   "version": "0.0.1",
+  "type": "module",
   "scripts": {
-    "develop": "gatsby develop",
-    "start": "gatsby develop",
-    "build": "gatsby build"
+    "dev": "astro dev",
+    "start": "astro dev",
+    "build": "astro build",
+    "preview": "astro preview"
   },
   "dependencies": {
-    "gatsby": "^2.13.65",
-    "gatsby-plugin-emotion": "^4.1.2",
-    "gatsby-plugin-mdx": "^1.0.24",
-    "gatsby-theme-blog": "^1.0.2",
-    "react": "^16.9.0",
-    "react-dom": "^16.9.0"
+    "astro": "^5.8.0",
+    "@astrojs/mdx": "^4.3.0",
+    "@astrojs/rss": "^4.0.0"
+  },
+  "devDependencies": {
+    "typescript": "^5.8.0"
   }
 }