add black-box debug harness

internal/harness/scenarios/switch_via_palette.json

@@ -0,0 +1,34 @@
+{
+  "name": "switch_via_palette",
+  "scripts": [
+    {
+      "name": "named-loop",
+      "body": "#!/bin/sh\necho \"$1 READY\"\nsleep 5\n"
+    }
+  ],
+  "steps": [
+    {
+      "type": "mcp_call",
+      "method": "spawn_process",
+      "params": { "kind": "command", "argv": ["named-loop", "first"], "name": "first" },
+      "save_as": "first"
+    },
+    {
+      "type": "mcp_call",
+      "method": "spawn_process",
+      "params": { "kind": "command", "argv": ["named-loop", "second"], "name": "second" },
+      "save_as": "second"
+    },
+    { "type": "wait_text", "contains": "second READY", "timeout_ms": 5000 },
+    { "type": "send_chord", "chord": "ctrl-k" },
+    { "type": "send_text", "text": "Switch to first" },
+    { "type": "send_chord", "chord": "enter" },
+    { "type": "wait_text", "contains": "first READY", "timeout_ms": 5000 },
+    {
+      "type": "assert_mcp",
+      "method": "get_project_status",
+      "path": "processes.0.name",
+      "equals": "first"
+    }
+  ]
+}