vars/interface: Change prompt.persist default to false

Change `prompt.persist` default to false.
We want a consistent default that is not conditionally dependent on
other values.
This makes communication on how the functionality is used more
consistent and easier understood.

nixosModules/clanCore/vars/interface.nix

@@ -282,7 +282,7 @@ in
                         ```
                       '';
                       type = bool;
-                      default = generator.config.script == "";
+                      default = false;
                     };
                     description = lib.mkOption {
                       description = ''

pkgs/clan-cli/tests/test_vars.py

@@ -600,6 +600,7 @@ def test_api_set_prompts(
     config["nixpkgs"]["hostPlatform"] = "x86_64-linux"
     my_generator = config["clan"]["core"]["vars"]["generators"]["my_generator"]
     my_generator["prompts"]["prompt1"]["type"] = "line"
+    my_generator["prompts"]["prompt1"]["persist"] = True
     my_generator["files"]["prompt1"]["secret"] = False
     flake.refresh()