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_references.py

@@ -192,38 +192,19 @@ def test_expand_git_diff_staged_and_log(sample_repo: Path):
     assert "VALUE = 2" in result.message
 
 
-def test_missing_file_becomes_warning(sample_repo: Path):
+def test_binary_and_missing_files_become_warnings(sample_repo: Path):
     from agent.context_references import preprocess_context_references
 
     result = preprocess_context_references(
-        "Check @file:nope.txt",
+        "Check @file:blob.bin and @file:nope.txt",
         cwd=sample_repo,
         context_length=100_000,
     )
 
     assert result.expanded
-    assert len(result.warnings) == 1
-    assert "not found" in result.message.lower()
-
-
-def test_binary_file_yields_actionable_block_not_a_dead_warning(sample_repo: Path):
-    from agent.context_references import preprocess_context_references
-
-    result = preprocess_context_references(
-        "Check @file:blob.bin",
-        cwd=sample_repo,
-        context_length=100_000,
-    )
-
-    assert result.expanded
-    # The whole point: a binary attachment must NOT degrade into a discouraging
-    # warning that makes the model give up — it gets an actionable content block.
-    assert not result.warnings
-    assert "blob.bin" in result.message
+    assert len(result.warnings) == 2
     assert "binary" in result.message.lower()
-    assert "not supported" not in result.message.lower()
-    # And it must point the agent at the file so it can act on it with tools.
-    assert str(sample_repo / "blob.bin") in result.message
+    assert "not found" in result.message.lower()
 
 
 def test_soft_budget_warns_and_hard_budget_refuses(sample_repo: Path):