opentui(phase3): launcher integration — HERMES_TUI_ENGINE dual-engine

hermes --tui launches the native OpenTUI engine (Bun) when
HERMES_TUI_ENGINE=opentui (env) or display.tui_engine=opentui (config);
Ink stays the default and the shipping path is untouched.

- _resolve_tui_engine() (env > config > ink); refuses opentui on
  Windows/Termux (no Bun) -> falls back to ink with a notice.
- _make_opentui_argv() -> [bun, src/entry.real.tsx] (no build step).
- _bun_bin() with HERMES_BUN override.
- Branch at top of _make_tui_argv BEFORE _ensure_tui_node (Bun-only host
  must not bootstrap Node).
- Gate _launch_tui NODE_OPTIONS/--max-old-space-size on engine==ink (Bun
  is JSC; the V8 flag errors/ignores).

Verified end-to-end via tmux: real hermes --tui -> Bun -> OpenTUI ->
real Python gateway streamed a real reply. No-flag default still ink.

hermes_cli/providers.py

@@ -492,10 +492,7 @@ def get_label(provider_id: str) -> str:
 
 def is_aggregator(provider: str) -> bool:
     """Return True when the provider is a multi-model aggregator."""
-    provider_norm = normalize_provider(provider or "")
-    if provider_norm.startswith("custom:"):
-        return True
-    pdef = get_provider(provider_norm)
+    pdef = get_provider(provider)
     return pdef.is_aggregator if pdef else False