add black-box debug harness

internal/harness/scenarios/mcp_send_input.json

@@ -0,0 +1,30 @@
+{
+  "name": "mcp_send_input",
+  "steps": [
+    {
+      "type": "mcp_call",
+      "method": "spawn_process",
+      "params": {
+        "kind": "command",
+        "argv": ["sh", "-lc", "read line; echo got:$line; sleep 5"],
+        "name": "reader"
+      },
+      "save_as": "proc"
+    },
+    {
+      "type": "mcp_call",
+      "method": "send_input",
+      "params": {
+        "process_id": "{{proc.process_id}}",
+        "kind": "text",
+        "text": "hello",
+        "submit": true,
+        "wait_ms": 250,
+        "tail_mode": "grid"
+      },
+      "save_as": "input"
+    },
+    { "type": "assert_saved", "from": "input", "path": "tail.content", "contains": "got:hello" },
+    { "type": "wait_text", "contains": "got:hello", "timeout_ms": 5000 }
+  ]
+}