fix: surface /agents nudge while delegate_task is in-flight (TUI + CLI)

The subagent spawn-observability overlay added a `(/agents)` hint, but
only on the standalone "Spawn tree" panel, gated behind `!inlineDelegateKey`
— it never showed for a single delegate_task call, and only appeared once
subagents had already registered. A nudge that arrives at the end (or only
after spawn) is useless for the actual goal: letting users open the live
monitor *while* delegation is running.

Surface it the moment delegation starts, on both surfaces:

TUI (ui-tui/src/components/thinking.tsx)
- Show `(/agents)` on any "Delegate Task" tool group as soon as it appears
  (in-flight, before any subagent registers), not gated on subagents
  already existing. Same `startsWith('Delegate Task')` predicate already
  used for delegateGroups.

CLI (agent/tool_executor.py)
- Append `· /agents to monitor` to the delegate spinner label, which is
  displayed for the full duration of the delegate_task call. The previous
  attempt put the hint on the completion line (get_cute_tool_message),
  which only renders after the call finishes — reverted.

TUI tsc clean (pre-existing execFileNoThrow type errors unrelated);
subagentTree 35/35; display.py reverted to upstream.

agent/tool_executor.py

@@ -753,10 +753,14 @@ def execute_tool_calls_sequential(agent, assistant_message, messages: list, effe
         elif function_name == "delegate_task":
             tasks_arg = function_args.get("tasks")
             if tasks_arg and isinstance(tasks_arg, list):
-                spinner_label = f"🔀 delegating {len(tasks_arg)} tasks"
+                spinner_label = f"🔀 delegating {len(tasks_arg)} tasks · (/agents to monitor)"
             else:
                 goal_preview = (function_args.get("goal") or "")[:30]
-                spinner_label = f"🔀 {goal_preview}" if goal_preview else "🔀 delegating"
+                spinner_label = (
+                    f"🔀 {goal_preview} · (/agents to monitor)"
+                    if goal_preview
+                    else "🔀 delegating · (/agents to monitor)"
+                )
             spinner = None
             if agent._should_emit_quiet_tool_messages() and agent._should_start_quiet_spinner():
                 face = random.choice(KawaiiSpinner.get_waiting_faces())