refactor: hide install-service from CLI help, keep as backward-compat alias

Removed install-service from argparse subparsers so it doesn't appear
in the usage line or help listing. Intercepted before parse_args() for
backward compatibility — prints deprecation warning and forwards to
service install.

muxplex/cli.py

@@ -548,14 +548,6 @@ def main() -> None:
     serve_parser = sub.add_parser("serve", help="Start the server (default)")
     _add_serve_flags(serve_parser)
 
-    svc = sub.add_parser(
-        "install-service",
-        help="Install as a background service (systemd on Linux, launchd on macOS)",
-    )
-    svc.add_argument(
-        "--system", action="store_true", help="System-wide (requires sudo)"
-    )
-
     service_parser = sub.add_parser(
         "service", help="Manage the muxplex background service"
     )
@@ -587,9 +579,8 @@ def main() -> None:
         help="Force reinstall even if already up to date",
     )
 
-    args = parser.parse_args()
-
-    if args.command == "install-service":
+    # Intercept deprecated 'install-service' before argparse sees it
+    if len(sys.argv) > 1 and sys.argv[1] == "install-service":
         print(
             "\u26a0 'muxplex install-service' is deprecated."
             " Use 'muxplex service install' instead.",
@@ -598,7 +589,11 @@ def main() -> None:
         from muxplex.service import service_install  # noqa: PLC0415
 
         service_install()
-    elif args.command == "show-password":
+        return
+
+    args = parser.parse_args()
+
+    if args.command == "show-password":
         show_password()
     elif args.command == "reset-secret":
         reset_secret()

muxplex/tests/test_cli.py

@@ -191,21 +191,20 @@ def test_reset_secret_prints_warning(tmp_path, monkeypatch, capsys):
     )
 
 
-def test_install_service_help_text_mentions_background_service():
-    """install-service help must mention 'service', not just 'systemd'."""
-    import io
+def test_install_service_deprecated_alias_prints_warning(capsys):
+    """install-service must print deprecation warning and forward to service install."""
     from muxplex.cli import main
 
-    buf = io.StringIO()
-    with patch("sys.argv", ["muxplex", "install-service", "--help"]):
-        try:
-            with patch("sys.stdout", buf):
+    with (
+        patch("sys.argv", ["muxplex", "install-service"]),
+        patch("muxplex.service.service_install") as mock_install,
+    ):
         main()
-        except SystemExit:
-            pass
 
-    help_text = buf.getvalue().lower()
-    assert "service" in help_text
+    captured = capsys.readouterr()
+    assert "deprecated" in captured.err.lower()
+    assert "service install" in captured.err.lower()
+    mock_install.assert_called_once()
 
 
 def test_check_dependencies_exits_when_ttyd_missing(monkeypatch):