chore(blog-search): add astro/check dep, plan, and follow-up note

- @astrojs/check + typescript bump are needed to run `npx astro check`,
  which is the type-verification command used throughout this branch.
- docs/superpowers/plans/ pairs with the existing spec under specs/.
- docs/superpowers/follow-ups/ records the featured-match behaviour
  change surfaced during manual test; addresses in a later branch.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

package.json

@@ -10,6 +10,7 @@
     "astro": "astro"
   },
   "dependencies": {
+    "@astrojs/check": "^0.9.9",
     "@astrojs/mdx": "^4.0.0",
     "@astrojs/react": "^4.0.0",
     "@astrojs/sitemap": "^3.2.1",
@@ -20,6 +21,6 @@
   "devDependencies": {
     "@types/react": "^18.3.12",
     "@types/react-dom": "^18.3.1",
-    "typescript": "^5.6.3"
+    "typescript": "^5.9.3"
   }
 }