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_lib/machines/machines.py

@@ -112,19 +112,7 @@ class Machine:
     def vars_generators(self) -> list["Generator"]:
         from clan_cli.vars.generate import Generator
 
-        try:
-            generators_data = self.select(
-                "config.clan.core.vars._serialized.generators"
-            )
-            if generators_data is None:
-                return []
-            _generators = [Generator.from_json(gen) for gen in generators_data.values()]
-            for gen in _generators:
-                gen.machine(self)
-        except Exception:
-            return []
-        else:
-            return _generators
+        return Generator.generators_from_flake(self.name, self.flake, self)
 
     @property
     def secrets_upload_directory(self) -> str: