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.

tools/checkpoint_manager.py

@@ -307,7 +307,6 @@ def _run_git(
             timeout=timeout,
             env=env,
             cwd=str(normalized_working_dir),
-            stdin=subprocess.DEVNULL,
         )
         ok = result.returncode == 0
         stdout = result.stdout.strip()
@@ -427,7 +426,6 @@ def _init_store(store: Path, working_dir: str) -> Optional[str]:
             ["git", "init", "--bare", str(store)],
             capture_output=True, text=True,
             env=init_env, timeout=_GIT_TIMEOUT,
-            stdin=subprocess.DEVNULL,
         )
         if result.returncode != 0:
             return f"Shadow store init failed: {result.stderr.strip()}"