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/var.py

@@ -1,6 +1,6 @@
 from dataclasses import dataclass
 from pathlib import Path
-from typing import TYPE_CHECKING, Any
+from typing import TYPE_CHECKING
 
 if TYPE_CHECKING:
     from clan_cli.vars.generate import Generator
@@ -67,16 +67,3 @@ class Var:
                 return f"{self.id}: ********"
             return f"{self.id}: {self.printable_value}"
         return f"{self.id}: <not set>"
-
-    @classmethod
-    def from_json(cls: type["Var"], generator_name: str, data: dict[str, Any]) -> "Var":
-        return cls(
-            id=f"{generator_name}/{data['name']}",
-            name=data["name"],
-            secret=data["secret"],
-            deploy=data["deploy"],
-            owner=data.get("owner", "root"),
-            group=data.get("group", "root"),
-            mode=int(data.get("mode", "0400"), 8),
-            needed_for=data.get("neededFor", "services"),
-        )