fix: install pnpm via corepack in CI

pnpm/action-setup's self-installer fails on Node 24 ("Running
self-installer..." exits 1), breaking both workflows. Use corepack
instead, which picks up the pnpm version from the packageManager field
and matches how the Dockerfile installs it.

Also move upload-artifact to v4, since v3 is retired.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

.github/workflows/security-audit.yml

@@ -29,9 +29,7 @@ jobs:
           node-version: '24'
 
       - name: Setup pnpm
-        uses: pnpm/action-setup@v4
-        with:
-          version: 10
+        run: corepack enable
 
       - name: Get pnpm store directory
         shell: bash
@@ -60,7 +58,7 @@ jobs:
         run: pnpm outdated || true
 
       - name: Upload security report
-        uses: actions/upload-artifact@v3
+        uses: actions/upload-artifact@v4
         if: always()
         with:
           name: security-report