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/gateway/test_session_api.py

@@ -75,54 +75,6 @@ async def test_capabilities_advertises_session_control_surface(adapter):
     }
 
 
-@pytest.mark.asyncio
-async def test_run_agent_binds_api_session_context_for_tool_env(adapter, monkeypatch):
-    """API-server request sessions should reach tools and terminal subprocess env."""
-    monkeypatch.setenv("HERMES_SESSION_ID", "stale-session")
-    observed = {}
-
-    class FakeAgent:
-        session_prompt_tokens = 0
-        session_completion_tokens = 0
-        session_total_tokens = 0
-
-        def __init__(self, session_id: str):
-            self.session_id = session_id
-
-        def run_conversation(self, user_message, conversation_history, task_id):
-            from gateway.session_context import get_session_env
-            from tools.environments.local import _make_run_env
-
-            observed["task_id"] = task_id
-            observed["context_session_id"] = get_session_env("HERMES_SESSION_ID")
-            observed["context_platform"] = get_session_env("HERMES_SESSION_PLATFORM")
-            observed["context_session_key"] = get_session_env("HERMES_SESSION_KEY")
-            observed["child_session_id"] = _make_run_env({}).get("HERMES_SESSION_ID")
-            return {"final_response": "ok"}
-
-    def fake_create_agent(**kwargs):
-        return FakeAgent(kwargs["session_id"])
-
-    monkeypatch.setattr(adapter, "_create_agent", fake_create_agent)
-
-    result, usage = await adapter._run_agent(
-        user_message="hello",
-        conversation_history=[],
-        session_id="request-session",
-        gateway_session_key="request-key",
-    )
-
-    assert result["session_id"] == "request-session"
-    assert usage == {"input_tokens": 0, "output_tokens": 0, "total_tokens": 0}
-    assert observed == {
-        "task_id": "request-session",
-        "context_session_id": "request-session",
-        "context_platform": "api_server",
-        "context_session_key": "request-key",
-        "child_session_id": "request-session",
-    }
-
-
 @pytest.mark.asyncio
 async def test_session_crud_and_message_history(adapter, session_db):
     app = _create_session_app(adapter)