fix(workflow): pin packageManager so pnpm/action-setup@v4 resolves

Without a packageManager field (or `version:` in the workflow),
pnpm/action-setup@v4 fails with "No pnpm version is specified" — which
broke every pnpm-using job on PR #10 (build, lint, test, typecheck,
commit-lint). Pinning packageManager fixes all of them and is the
canonical way per Corepack/pnpm guidance.

Refs #9

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: abdout

package.json

@@ -3,6 +3,7 @@
   "version": "1.0.0",
   "description": "Remote AI Development Infrastructure - كن (Be!)",
   "private": true,
+  "packageManager": "pnpm@9.15.0",
   "scripts": {
     "dev": "next dev",
     "dev:turbo": "next dev --turbopack",