M2: content collections — case studies, blog, RSS, tags, sitemap

Projects + blog as schema-validated content collections; structured case
studies (problem/design/outcome), blog with tag pages, reading time, RSS
feed (drafts excluded), sitemap, and Shiki dual-theme code highlighting.

package.json

@@ -14,6 +14,8 @@
     "astro": "astro"
   },
   "dependencies": {
+    "@astrojs/rss": "^4.0.18",
+    "@astrojs/sitemap": "^3.7.3",
     "@tailwindcss/vite": "^4.3.1",
     "astro": "^6.4.7",
     "tailwindcss": "^4.3.1"