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.

website/docs/developer-guide/model-provider-plugin.md

@@ -131,9 +131,8 @@ class AcmeProfile(ProviderProfile):
 
     def build_api_kwargs_extras(self, *, reasoning_config=None, **context):
         """Returns (extra_body_additions, top_level_kwargs). Needed when some
-        fields go top-level (Kimi's reasoning_effort, OpenRouter's verbosity for
-        adaptive Anthropic models) and some go in extra_body (OpenRouter's
-        reasoning dict). Default: ({}, {})."""
+        fields go top-level (Kimi's reasoning_effort) and some go in extra_body
+        (OpenRouter's reasoning dict). Default: ({}, {})."""
         return {}, {}
 
     def fetch_models(self, *, api_key=None, timeout=8.0) -> list[str] | None: