refactor: remove _serialized field and implement efficient vars selection

- Remove _serialized field from vars interface to prevent serialization
  errors with throwing passBackend field
- Implement direct selection of generator fields using multi-select syntax
- Refactor vars_generators() to use new Generator.from_flake() method that
  selects only safe fields (avoiding non-serializable values)
- Remove unused legacy methods: Generator.from_json(), Var.from_json(),
  Prompt.from_json()
- Update precaching to match new selection approach

This fixes the serialization errors that were preventing vars from working
with the new password-store implementation by avoiding the problematic
_serialized field entirely.

pkgs/clan-cli/clan_cli/vars/prompt.py

@@ -32,12 +32,12 @@ class Prompt:
     previous_value: str | None = None
 
     @classmethod
-    def from_json(cls: type["Prompt"], data: dict[str, Any]) -> "Prompt":
+    def from_nix(cls: type["Prompt"], data: dict[str, Any]) -> "Prompt":
         return cls(
             name=data["name"],
-            description=data["description"],
-            prompt_type=PromptType(data["type"]),
-            persist=data.get("persist", data["persist"]),
+            description=data.get("description", data["name"]),
+            prompt_type=PromptType(data.get("type", "line")),
+            persist=data.get("persist", False),
         )