fix(docs): vercel build filter by name (cairn-*), not path

Vercel's Root Directory is apps/docs-site, so the `./packages/*` path glob
resolved to apps/docs-site/packages/* (nothing) and built no workspace
packages → the docs build failed importing them. Name filters are cwd-
independent: `--filter "cairn-*"` builds all seven from any directory.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: tomymaritano

apps/docs-site/vercel.json

@@ -1,5 +1,5 @@
 {
   "$schema": "https://openapi.vercel.sh/vercel.json",
   "framework": "nextjs",
-  "buildCommand": "pnpm --filter \"./packages/*\" build && pnpm --filter @cairn/docs build"
+  "buildCommand": "pnpm --filter \"cairn-*\" build && pnpm --filter @cairn/docs build"
 }