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/user-guide/messaging/slack.md

@@ -280,11 +280,6 @@ thread.
 Only the first token is checked against the known command list, so
 casual messages like `!nice work` pass through to the agent unchanged.
 
-Approval prompts (dangerous command / `execute_code` approval) normally
-render as interactive buttons. When buttons can't be delivered and
-Hermes falls back to a text prompt, the prompt instructs you to reply
-with `!approve` / `!deny` — the form that works inside threads.
-
 ### Advanced: emit only the slash-commands array
 
 If you maintain your Slack manifest by hand and just want the slash