Add stackable toast notifications

Replaces the single-slot status-line flash with a top-right toast
stack over the focused pane. flashError, flashTransient, and
notifyAttention all push onto the same stack (cap 5, FIFO drop).
Ctrl-N dismisses the most recent toast; empty stack falls through to
the focused PTY so readline / nano / emacs / opencode bindings keep
working. A new "Clear notifications" palette item empties the stack.

internal/harness/scenarios/toast_dismiss.json

@@ -0,0 +1,32 @@
+{
+  "name": "toast_dismiss",
+  "presets": {
+    "processes": [
+      {
+        "name": "steady",
+        "argv": ["sh", "-lc", "printf 'STEADY READY\\n'; sleep 30"]
+      }
+    ]
+  },
+  "trust": ["steady"],
+  "steps": [
+    {
+      "type": "mcp_call",
+      "method": "spawn_process",
+      "params": {"kind": "command", "preset": "steady", "name": "steady"},
+      "save_as": "proc"
+    },
+    { "type": "wait_text", "contains": "STEADY READY", "timeout_ms": 5000 },
+    {
+      "type": "mcp_call",
+      "method": "request_human_attention",
+      "params": {"process_id": "{{proc.process_id}}", "reason": "needs eyes on the deploy"}
+    },
+    { "type": "wait_text", "contains": "needs eyes on the deploy", "timeout_ms": 5000 },
+    { "type": "assert_contains", "contains": "STEADY READY" },
+    { "type": "send_chord", "chord": "ctrl-n" },
+    { "type": "wait_stable", "timeout_ms": 2000 },
+    { "type": "assert_contains", "contains": "STEADY READY" },
+    { "type": "assert_not_contains", "contains": "needs eyes on the deploy" }
+  ]
+}