fix: add rewrite rule so install.cmem.ai root serves install.sh

Without this, curl https://install.cmem.ai returns 404 because
Vercel has no index file mapping for the root path.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

install/vercel.json

@@ -1,5 +1,8 @@
 {
   "$schema": "https://openapi.vercel.sh/vercel.json",
+  "rewrites": [
+    { "source": "/", "destination": "/install.sh" }
+  ],
   "headers": [
     {
       "source": "/(.*)\\.sh",