feat(tui): nudge toward /agents dashboard when delegation starts

The TUI already ships a rich /agents spawn-tree dashboard (live tree,
timeline, per-child tokens/cost/files/tools, kill/pause), but nothing
surfaced it — during delegation the transcript stayed quiet and users
had to already know to type /agents.

Drop a one-time transient activity hint ("subagents working · /agents
to watch live") the first time a turn starts delegating, matching the
existing "· /logs to inspect" house style. Guards keep it unobtrusive:

- fires at most once per turn (resets on message.start)
- silent when the /agents overlay is already open
- gated by display.tui_agents_nudge (default true)

Hooked on subagent.start, not subagent.spawn_requested: the delegate
progress callback in tools/delegate_tool.py only relays start/complete
to the gateway and drops spawn_requested, so start is the first
delegation event the TUI reliably receives. spawn_requested is wired
too for the future case, guarded once-per-turn.

Adds the display.tui_agents_nudge config default and gatewayTypes entry.

hermes_cli/config.py

@@ -1183,6 +1183,11 @@ DEFAULT_CONFIG = {
         # Mirrors `hermes -c` muscle memory.  Default off so existing
         # users aren't surprised.  HERMES_TUI_RESUME=<id> always wins.
         "tui_auto_resume_recent": False,
+        # When true (default), `hermes --tui` drops a one-time hint
+        # ("subagents working · /agents to watch live") the first time a turn
+        # starts delegating, nudging the user toward the live spawn-tree
+        # dashboard. Set false to suppress the hint.
+        "tui_agents_nudge": True,
         "bell_on_complete": False,
         "show_reasoning": False,
         "streaming": False,