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_context_compressor.py

@@ -3,11 +3,7 @@
 import pytest
 from unittest.mock import patch, MagicMock
 
-from agent.context_compressor import (
-    ContextCompressor,
-    HISTORICAL_TASK_HEADING,
-    SUMMARY_PREFIX,
-)
+from agent.context_compressor import ContextCompressor, SUMMARY_PREFIX
 
 
 @pytest.fixture()
@@ -161,7 +157,7 @@ class TestCompress:
             result = c.compress(msgs)
 
         combined = "\n".join(str(m.get("content", "")) for m in result)
-        assert HISTORICAL_TASK_HEADING in combined
+        assert "## Active Task" in combined
         assert "Please fix the compression summary failure" in combined
         assert "read_file" in combined
         assert "agent/context_compressor.py" in combined
@@ -1217,8 +1213,7 @@ class TestCompressWithClient:
         """When the summary lands as standalone role='user' (e.g. head ends
         with assistant/tool), the message body must include the explicit
         '--- END OF CONTEXT SUMMARY ---' marker. Without it, weak models
-        read the verbatim past user request quoted in the historical task
-        snapshot as
+        read the verbatim past user request quoted in '## Active Task' as
         fresh input (#11475, #14521).
         """
         mock_response = MagicMock()