test(gateway): make test_gateway collectable on Windows

`import pty` at module scope pulls in `termios`, which does not exist on
Windows. That raised ModuleNotFoundError during *collection*, so pytest
aborted the whole module with

    Interrupted: 1 error during collection

before any skip marker could take effect. The single PTY-dependent test
was already correctly marked `skipif(sys.platform == "win32")` — the
import crashed ahead of it and took the module's 13 other, entirely
platform-agnostic tests down as collateral. Windows contributors got zero
gateway coverage and, worse, a collection error that masks real failures
in any batch that includes this file.

Two changes:

- Move `import pty` into the `stdin_is_tty` branch that actually uses it
  (the sole `pty.openpty()` call). Nothing else in the module needs it.
- Skip `test_systemd_install_checks_linger_status` on Windows. It drives
  `_systemd_linger_enabled()` -> `os.getuid()`, which does not exist on
  Windows; the production helper is annotated
  "windows-footgun: ok — POSIX systemd helper, never invoked on Windows",
  so the test is Linux-only by nature. It was previously hidden behind
  the collection crash.

POSIX behaviour is unchanged: both guards are `skipif(win32)`, inactive
off Windows, and the local import resolves exactly where the module-level
one did.

    before (Windows): 0 collected, 1 collection error
    after  (Windows): 14 collected, 9 passed, 5 skipped

tests/hermes_cli/test_gateway.py

@@ -2,7 +2,6 @@
 
 import argparse
 import os
-import pty
 import signal
 import subprocess
 import sys
@@ -111,6 +110,13 @@ def test_gateway_run_subprocess_preserves_daemon_exit_codes(
     master_fd = slave_fd = None
     try:
         if stdin_is_tty:
+            # Imported here, not at module scope: ``pty`` pulls in ``termios``,
+            # which does not exist on Windows, so a top-level import raises
+            # ModuleNotFoundError during *collection* — before the skipif above
+            # can take effect — and takes the whole module's Windows-viable
+            # tests down with it.
+            import pty
+
             master_fd, slave_fd = pty.openpty()
             stdin = slave_fd
         else:
@@ -219,6 +225,10 @@ class TestContainerSystemdSupport:
 
 
 
+@pytest.mark.skipif(
+    sys.platform == "win32",
+    reason="systemd user-linger is Linux-only (drives os.getuid())",
+)
 def test_systemd_install_checks_linger_status(monkeypatch, tmp_path, capsys):
     unit_path = tmp_path / "systemd" / "user" / "hermes-gateway.service"