fix(a2ui-renderer): remove redundant prepack script causing canary publish failures

The prepack script ran `pnpm run build` during `pnpm publish`, duplicating
the build that prerelease.ts/publish-release.ts already do across the
workspace. With tsdown's `exports: true` regenerating package.json's
exports field on every build, the second rewrite happened mid-publish
and pnpm's pack step failed with ENOENT on the staging tarball.

No other monorepo package has prepack — bringing a2ui-renderer in line.

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

Author: tylerslaton

packages/a2ui-renderer/package.json

@@ -38,7 +38,6 @@
     "access": "public"
   },
   "scripts": {
-    "prepack": "pnpm run build",
     "build": "tsdown",
     "check-types": "tsc --noEmit -p tsconfig.json",
     "test": "vitest run",