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/agent/test_system_prompt.py

@@ -55,44 +55,3 @@ class TestContextFileCwd:
     def test_configured_dir_when_terminal_cwd_set(self, monkeypatch, tmp_path):
         monkeypatch.setenv("TERMINAL_CWD", str(tmp_path))
         assert _captured_context_cwd(_make_agent()) == tmp_path
-
-
-def _stable_prompt(agent):
-    with (
-        patch("run_agent.load_soul_md", return_value=""),
-        patch("run_agent.build_nous_subscription_prompt", return_value=""),
-        patch("run_agent.build_environment_hints", return_value=""),
-        patch("run_agent.build_context_files_prompt", return_value=""),
-    ):
-        return build_system_prompt_parts(agent)["stable"]
-
-
-class TestCodingContextBlock:
-    def test_injected_when_active(self, monkeypatch, tmp_path):
-        import subprocess
-
-        subprocess.run(["git", "-C", str(tmp_path), "init", "-q"], check=True)
-        monkeypatch.setenv("TERMINAL_CWD", str(tmp_path))
-        agent = _make_agent(valid_tool_names=["read_file"], platform="cli")
-        stable = _stable_prompt(agent)
-        assert "coding agent" in stable
-        assert "Workspace" in stable
-
-    def test_absent_when_off(self, monkeypatch, tmp_path):
-        import subprocess
-
-        subprocess.run(["git", "-C", str(tmp_path), "init", "-q"], check=True)
-        monkeypatch.setenv("TERMINAL_CWD", str(tmp_path))
-        agent = _make_agent(valid_tool_names=["read_file"], platform="cli")
-        # Drive the real path: force the resolved mode to "off" via config.
-        with patch("agent.coding_context._coding_mode", return_value="off"):
-            stable = _stable_prompt(agent)
-        assert "coding agent" not in stable
-
-    def test_absent_without_tools(self, monkeypatch, tmp_path):
-        import subprocess
-
-        subprocess.run(["git", "-C", str(tmp_path), "init", "-q"], check=True)
-        monkeypatch.setenv("TERMINAL_CWD", str(tmp_path))
-        agent = _make_agent(valid_tool_names=[], platform="cli")
-        assert "coding agent" not in _stable_prompt(agent)