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/

.gitignore

@@ -54,9 +54,9 @@ typings/
 # dotenv environment variables file
 .env
 
-# gatsby files
-.cache/
-public
+# astro
+dist/
+.astro/
 
 # Mac files
 .DS_Store