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/environments/singularity.py

@@ -46,7 +46,6 @@ def _ensure_singularity_available() -> str:
     try:
         result = subprocess.run(
             [exe, "version"], capture_output=True, text=True, timeout=10,
-            stdin=subprocess.DEVNULL,
         )
     except FileNotFoundError:
         raise RuntimeError(
@@ -137,7 +136,6 @@ def _get_or_build_sif(image: str, executable: str = "apptainer") -> str:
             result = subprocess.run(
                 [executable, "build", str(sif_path), image],
                 capture_output=True, text=True, timeout=600, env=env,
-                stdin=subprocess.DEVNULL,
             )
             if result.returncode != 0:
                 logger.warning("SIF build failed, falling back to docker:// URL")
@@ -220,7 +218,7 @@ class SingularityEnvironment(BaseEnvironment):
         cmd.extend([str(self.image), self.instance_id])
 
         try:
-            result = subprocess.run(cmd, capture_output=True, text=True, timeout=120, stdin=subprocess.DEVNULL)
+            result = subprocess.run(cmd, capture_output=True, text=True, timeout=120)
             if result.returncode != 0:
                 raise RuntimeError(f"Failed to start instance: {result.stderr}")
             self._instance_started = True
@@ -252,7 +250,6 @@ class SingularityEnvironment(BaseEnvironment):
                 subprocess.run(
                     [self.executable, "instance", "stop", self.instance_id],
                     capture_output=True, text=True, timeout=30,
-                    stdin=subprocess.DEVNULL,
                 )
                 logger.info("Singularity instance %s stopped", self.instance_id)
             except Exception as e: