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.

ui-tui/src/components/thinking.tsx

@@ -1073,6 +1073,10 @@ export const ToolTrail = memo(function ToolTrail({
             const branch: TreeBranch = index === groups.length - 1 ? 'last' : 'mid'
             const childRails = nextTreeRails(rails, branch)
             const hasInlineSubagents = inlineDelegateKey === group.key
+            // Surface the /agents hint the moment a delegate group appears —
+            // while it's still in-flight and before any subagent has
+            // registered — so users can open the live monitor immediately.
+            const isDelegateGroup = group.label.startsWith('Delegate Task')
 
             return (
               <Box flexDirection="column" key={group.key}>
@@ -1083,6 +1087,11 @@ export const ToolTrail = memo(function ToolTrail({
                     <>
                       <Text color={t.color.accent}>● </Text>
                       {toolLabel(group)}
+                      {isDelegateGroup ? (
+                        <Text color={t.color.statusFg} dim>
+                          {'  (/agents to monitor)'}
+                        </Text>
+                      ) : null}
                     </>
                   }
                   rails={rails}