ci(sandbox): use docker_isolation.mode (global key) + assert sandbox actually resolved

CodexReviewer caught the probe was vacuous: the config used a top-level
"isolation" key, but the GLOBAL isolation mode is docker_isolation.mode
(per-server isolation is the only 'isolation' key). The wrong key was silently
ignored, so the server started with isolation_mode=none — the 'sandbox' probe
never tested sandbox.

- workflow + harness: isolation -> docker_isolation for the global mode
- workflow: assert the server log shows isolation_mode=sandbox (fail if not),
  so a future wrong-key regression can't pass vacuously
- harness positive case now actually runs the stdio 'everything' server under
  Landlock (inherits global sandbox); negative baseline under docker (AppArmor)

Related #71

Author: Dumbris

.github/workflows/sandbox-integration.yml

@@ -96,7 +96,7 @@ jobs:
             "listen": "127.0.0.1:19237",
             "api_key": "qa-sandbox-ci-test",
             "enable_web_ui": false,
-            "isolation": { "mode": "sandbox" },
+            "docker_isolation": { "mode": "sandbox" },
             "mcpServers": []
           }
           EOF
@@ -137,7 +137,15 @@ jobs:
             cat /tmp/mcp3236-ci/server.log
             exit 1
           fi
-          echo "Server healthy (/readyz) with isolation.mode=sandbox"
+          # Prove the server actually resolved SANDBOX mode (the global key is
+          # docker_isolation.mode — a wrong key silently falls back to "none",
+          # which would make this probe vacuous).
+          if ! grep -i "isolation_mode" /tmp/mcp3236-ci/server.log | grep -qi "sandbox"; then
+            echo "ERROR: server did not start in sandbox mode (expected isolation_mode=sandbox)"
+            grep -i "isolation_mode" /tmp/mcp3236-ci/server.log || echo "(no isolation_mode log line found)"
+            exit 1
+          fi
+          echo "Server healthy (/readyz) and confirmed isolation_mode=sandbox"
 
       - name: macOS/non-Linux graceful-degrade probe (build check)
         run: |

docs/development/sandbox-snap-docker-harness.md

@@ -44,7 +44,7 @@ cat > /tmp/harness-docker/mcp_config.json <<'EOF'
   "listen": "127.0.0.1:18080",
   "api_key": "harness-key",
   "enable_web_ui": false,
-  "isolation": { "mode": "docker" },
+  "docker_isolation": { "mode": "docker" },
   "mcpServers": [
     {
       "name": "everything",
@@ -99,7 +99,7 @@ cat > /tmp/harness-sandbox/mcp_config.json <<'EOF'
   "listen": "127.0.0.1:18081",
   "api_key": "harness-key",
   "enable_web_ui": false,
-  "isolation": { "mode": "sandbox" },
+  "docker_isolation": { "mode": "sandbox" },
   "mcpServers": [
     {
       "name": "everything",