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/tests/test_secrets_generate.py

@@ -107,7 +107,7 @@ def test_generate_secret(
     )
 
     assert store2.exists("", "age.key")
-    (
+    assert (
         test_flake_with_core.path
         / "vars"
         / "per-machine"