make host key check an enum instead of an literal type

this is more typesafe at runtime.

pkgs/clan-cli/clan_cli/tests/hosts.py

@@ -4,6 +4,7 @@ from pathlib import Path
 
 import pytest
 from clan_cli.tests.sshd import Sshd
+from clan_lib.ssh.host_key import HostKeyCheck
 from clan_lib.ssh.remote import Remote
 
 
@@ -16,7 +17,7 @@ def hosts(sshd: Sshd) -> list[Remote]:
             port=sshd.port,
             user=login,
             private_key=Path(sshd.key),
-            host_key_check="none",
+            host_key_check=HostKeyCheck.NONE,
             command_prefix="local_test",
         )
     ]