ci: shellcheck en GitHub Actions

- Workflow en push/PR: shellcheck --severity=error sobre *.sh, core/*.sh y modules/*.sh
- El real check corre en GitHub; local sin shellcheck instalado (documentado)

.github/workflows/shellcheck.yml

@@ -0,0 +1,16 @@
+name: shellcheck
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+jobs:
+  shellcheck:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Run shellcheck
+        run: shellcheck --severity=error *.sh core/*.sh modules/*.sh