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/tools/test_send_message_tool.py

@@ -1199,11 +1199,6 @@ class TestParseTargetRefE164:
         assert chat_id == "+15551234567"
         assert is_explicit is True
 
-    def test_photon_e164_is_explicit(self):
-        chat_id, _, is_explicit = _parse_target_ref("photon", "+15551234567")
-        assert chat_id == "+15551234567"
-        assert is_explicit is True
-
     def test_signal_bare_digits_still_work(self):
         """Bare digit strings continue to match the generic numeric branch."""
         chat_id, _, is_explicit = _parse_target_ref("signal", "15551234567")