add black-box debug harness

internal/harness/scenarios/trust_required.json

@@ -0,0 +1,19 @@
+{
+  "name": "trust_required",
+  "presets": {
+    "processes": [
+      {
+        "name": "untrusted",
+        "argv": ["sh", "-lc", "echo should-not-run"]
+      }
+    ]
+  },
+  "steps": [
+    {
+      "type": "assert_mcp",
+      "method": "spawn_process",
+      "params": { "kind": "command", "preset": "untrusted" },
+      "error_kind": "needs_trust"
+    }
+  ]
+}