test: add pytest guard for subprocess stdin= in TUI-context code

Wraps scripts/check_subprocess_stdin.py as a pytest so CI catches
regressions when new subprocess calls are added without stdin=.

tests/tools/test_subprocess_stdin_guard.py

@@ -0,0 +1,25 @@
+"""Verify that TUI-context subprocess calls specify stdin=.
+
+This is the pytest wrapper for scripts/check_subprocess_stdin.py.
+It runs as part of the test suite so CI catches regressions when new
+subprocess calls are added without stdin=subprocess.DEVNULL.
+"""
+
+import subprocess
+import sys
+from pathlib import Path
+
+SCRIPT = Path(__file__).resolve().parents[2] / "scripts" / "check_subprocess_stdin.py"
+
+
+def test_all_tui_subprocess_calls_have_stdin():
+    """Every subprocess.run/Popen in TUI-context code must set stdin=."""
+    result = subprocess.run(
+        [sys.executable, str(SCRIPT)],
+        capture_output=True,
+        text=True,
+        timeout=30,
+    )
+    assert result.returncode == 0, (
+        f"subprocess stdin= check failed:\n{result.stdout}\n{result.stderr}"
+    )