Fix Gitea runner CI: remove sudo, install system deps via apt, replace setup-python with apt-get

.github/workflows/validate-all.yml

@@ -26,9 +26,9 @@ jobs:
             package-lock.json
 
       - name: Setup Python
-        uses: actions/setup-python@v5
-        with:
-          python-version: '3.12'
+        run: |
+          apt-get update
+          apt-get install -y python3 python3-pip
 
       - name: Enforce feature spec policy (PR only)
         if: github.event_name == 'pull_request'