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/subcommands/mcp.py

@@ -6,7 +6,6 @@ Handler injected to avoid importing ``main``.
 
 from __future__ import annotations
 
-import argparse
 from typing import Callable
 
 from hermes_cli.subcommands._shared import add_accept_hooks_flag
@@ -53,10 +52,7 @@ def build_mcp_parser(subparsers, *, cmd_mcp: Callable) -> None:
         "--command", dest="mcp_command", help="Stdio command (e.g. npx)"
     )
     mcp_add_p.add_argument(
-        "--args",
-        nargs=argparse.REMAINDER,
-        default=[],
-        help="Arguments for stdio command; must be the last option",
+        "--args", nargs="*", default=[], help="Arguments for stdio command"
     )
     mcp_add_p.add_argument("--auth", choices=["oauth", "header"], help="Auth method")
     mcp_add_p.add_argument("--preset", help="Known MCP preset name")