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.

tests/hermes_cli/test_model_switch_custom_providers.py

@@ -65,15 +65,6 @@ def test_resolve_provider_full_finds_named_custom_provider():
     assert resolved.source == "user-config"
 
 
-def test_is_aggregator_recognizes_named_custom_provider():
-    assert providers_mod.is_aggregator("custom:hpc-ai") is True
-    assert providers_mod.is_aggregator("custom:litellm") is True
-
-
-def test_is_aggregator_leaves_unknown_provider_non_aggregator():
-    assert providers_mod.is_aggregator("not-a-provider") is False
-
-
 def test_switch_model_accepts_explicit_named_custom_provider(monkeypatch):
     """Shared /model switch pipeline should accept --provider for custom_providers."""
     monkeypatch.setattr(