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_credits_cold_start.py

@@ -123,29 +123,22 @@ def test_dev_fixtures_drive_cold_start():
 
 class _FakeAgent:
     """Minimal agent surface for the seed helper: state slots + an emit that runs
-    the real policy against the latch (mirroring run_agent._emit_credits_notices,
-    including the free-model suppression flag)."""
+    the real policy against the latch."""
 
-    def __init__(self, provider="nous", model=""):
-        from agent.credits_tracker import evaluate_credits_notices, is_free_tier_model
+    def __init__(self, provider="nous"):
+        from agent.credits_tracker import evaluate_credits_notices
 
         self.provider = provider
-        self.model = model
         self._credits_state = None
         self._credits_session_start_micros = None
         self._credits_latch = {"active": set(), "seen_below_90": False, "usage_band": None}
         self.emitted: list = []
         self._eval = evaluate_credits_notices
-        self._is_free = is_free_tier_model
 
     def _emit_credits_notices(self):
         if self._credits_state is None:
             return
-        show, clear = self._eval(
-            self._credits_state,
-            self._credits_latch,
-            model_is_free=self._is_free(self.model),
-        )
+        show, clear = self._eval(self._credits_state, self._credits_latch)
         self.emitted.append(([n.key for n in show], clear))
 
 
@@ -176,14 +169,6 @@ def test_seed_fires_depleted_at_session_open():
     assert a.emitted == [(["credits.depleted"], [])]
 
 
-def test_seed_depleted_suppressed_on_free_model():
-    """A session that opens depleted but on a Nous ``:free`` model must NOT show
-    the depleted banner — inference works fine on the free tier."""
-    a = _FakeAgent(model="nvidia/nemotron-3-ultra:free")
-    assert _seed(a, "depleted") is True
-    assert a.emitted == [([], [])]
-
-
 def test_seed_healthy_no_notice():
     a = _FakeAgent()
     assert _seed(a, "healthy") is True