test: add deprecation warning test for install-service subcommand

- Add test_install_service_subcommand_prints_deprecation_warning to
  verify that 'muxplex install-service' prints a deprecation warning
  to stderr containing 'deprecated' and 'muxplex service install'
- Update the deprecation warning message in main() to include
  'muxplex service install' as the recommended replacement command

Refs: task-4

muxplex/cli.py

@@ -723,7 +723,8 @@ def main() -> None:
         import sys
 
         print(
-            "Warning: 'install-service' is deprecated and will be removed in a future version.",
+            "Warning: 'install-service' is deprecated and will be removed in a future version."
+            " Use 'muxplex service install' instead.",
             file=sys.stderr,
         )
         install_service(system=args.system)

muxplex/tests/test_cli.py

@@ -969,6 +969,19 @@ def test_main_serve_subcommand_accepts_flags():
         )
 
 
+def test_install_service_subcommand_prints_deprecation_warning(capsys):
+    """'muxplex install-service' must print a deprecation warning to stderr."""
+    from muxplex.cli import main
+
+    with patch("muxplex.cli.install_service"):
+        with patch("sys.argv", ["muxplex", "install-service"]):
+            main()
+
+    captured = capsys.readouterr()
+    assert "deprecated" in captured.err.lower()
+    assert "muxplex service install" in captured.err
+
+
 def test_help_shows_single_upgrade_line():
     """Help output shows 'upgrade (update)' alias notation, not two separate subcommand entries."""
     import io