Run source contracts in disposable CI lane

Assistant: codex
Assistant-Model: gpt-5.6-sol
Assistant-Session: 01a02994-7685-7940-bf34-3555b8256018

.forgejo/workflows/ci-smoke.yaml

@@ -10,10 +10,18 @@ on:
 
 jobs:
   source-contract:
-    runs-on: self-hosted
+    # The host lane is intentionally minimal and does not provide the Node
+    # runtime required by actions/checkout. Run source validation in the
+    # documented disposable container lane instead.
+    runs-on: ubuntu-latest
     steps:
       - name: Check out source
         uses: actions/checkout@v4
+      - name: Prepare source-contract runtime
+        run: |
+          set -eu
+          apt-get update
+          apt-get install -y --no-install-recommends make python3 python3-yaml
       - name: Validate S1 source contracts
         run: |
           set -eu