fix(config): persist only the alias the runtime settings read

pydantic-settings takes the first alias present in the environment, even
when it is empty. persist_current() must save that same alias, so an empty
LLM_API_KEY does not let a non-empty OPENAI_API_KEY sibling land in the
file and restore a credential the run did not use.

strix/config/loader.py

@@ -79,16 +79,13 @@ def persist_current() -> None:
             continue
         for finfo in type(sub_model).model_fields.values():
             aliases = [alias.upper() for alias in _aliases_for(finfo)]
-            set_aliases = [alias for alias in aliases if os.environ.get(alias) is not None]
-            if not set_aliases:
+            active = next((alias for alias in aliases if alias in os.environ), None)
+            if active is None:
                 continue
             for alias in aliases:
                 env_block.pop(alias, None)
-            for alias in set_aliases:
-                value = os.environ[alias]
-                if value:
-                    env_block[alias] = value
-                    break
+            if os.environ[active]:
+                env_block[active] = os.environ[active]
 
     write_secret_text(target, json.dumps({"env": env_block}, indent=2))
 

tests/test_config_loader.py

@@ -364,6 +364,22 @@ def test_persist_current_empty_env_clears_file_value(
     assert json.loads(target.read_text(encoding="utf-8")) == {"env": {"STRIX_LLM": "file-model"}}
 
 
+def test_persist_current_empty_primary_alias_does_not_save_sibling(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    target = tmp_path / "cli-config.json"
+    target.write_text(json.dumps({"env": {"PERPLEXITY_API_KEY": "pplx"}}), encoding="utf-8")
+    loader.apply_config_override(target)
+    monkeypatch.setenv("LLM_API_KEY", "")
+    monkeypatch.setenv("OPENAI_API_KEY", "sibling-key")
+
+    assert loader.load_settings().llm.api_key == ""
+
+    loader.persist_current()
+
+    assert json.loads(target.read_text(encoding="utf-8")) == {"env": {"PERPLEXITY_API_KEY": "pplx"}}
+
+
 def test_persist_current_replaces_corrupt_file(
     tmp_path: Path, monkeypatch: pytest.MonkeyPatch
 ) -> None: