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.

gateway/session_context.py

@@ -106,7 +106,6 @@ def set_session_vars(
     user_id: str = "",
     user_name: str = "",
     session_key: str = "",
-    session_id: str = "",
     message_id: str = "",
     cwd: str = "",
 ) -> list:
@@ -128,7 +127,6 @@ def set_session_vars(
         _SESSION_USER_ID.set(user_id),
         _SESSION_USER_NAME.set(user_name),
         _SESSION_KEY.set(session_key),
-        _SESSION_ID.set(session_id),
         _SESSION_MESSAGE_ID.set(message_id),
     ]
     try:
@@ -159,7 +157,6 @@ def clear_session_vars(tokens: list) -> None:
         _SESSION_USER_ID,
         _SESSION_USER_NAME,
         _SESSION_KEY,
-        _SESSION_ID,
         _SESSION_MESSAGE_ID,
     ):
         var.set("")