fix(ci): use --frozen-lockfile in e2e workflow

Align with e2e.yml and ensure CI tests exactly the committed
dependency tree. The pr-policy job already blocks lockfile changes
in PRs, so frozen-lockfile is safe here.

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

.github/workflows/pr-e2e.yml

@@ -30,7 +30,7 @@ jobs:
           cache: pnpm
 
       - name: Install dependencies
-        run: pnpm install --no-frozen-lockfile
+        run: pnpm install --frozen-lockfile
 
       - name: Build
         run: pnpm build