ci(release): restore runtime assets preflight check

.github/workflows/full-release-validation.yml

@@ -240,6 +240,16 @@ jobs:
           fetch-depth: 1
           persist-credentials: true
 
+      - name: Verify Docker runtime-assets prune path
+        env:
+          DOCKER_BUILDKIT: "1"
+        run: |
+          set -euo pipefail
+          timeout --kill-after=30s 35m docker build \
+            --target runtime-assets \
+            --build-arg OPENCLAW_EXTENSIONS="diagnostics-otel,codex" \
+            .
+
       - name: Build and smoke test final Docker runtime image
         env:
           DOCKER_BUILDKIT: "1"