fix: use npm ci on Vercel so patch-package runs against pristine node_modules (#62)

Vercel restores node_modules from the build cache before running the
install command. With `npm install`, the cached files are kept in place
and patch-package then runs against a pre-patched starlight-llms-txt,
causing the postinstall step to fail with 'Failed to apply patch for
package starlight-llms-txt' on every deploy after #51.

`npm ci` deletes node_modules before installing, so patch-package
always runs against the pristine npm tarball.

Co-authored-by: Oz <oz-agent@warp.dev>

Author: hongyi-chen

vercel.json

@@ -1,6 +1,6 @@
 {
   "framework": "astro",
-  "installCommand": "npm install",
+  "installCommand": "npm ci",
   "buildCommand": "npm run build",
   "devCommand": "npm run dev",
   "headers": [