fix: --config flag now fully overrides ~/.strix/cli-config.json (#457)

* fix: --config flag now fully overrides ~/.strix/cli-config.json (fixes #377)

Previously, env vars applied from the default config at module import time
were not cleared when --config was later processed, causing settings from
~/.strix/cli-config.json to leak into runs that specified a custom config.

Track which vars were applied by the initial default-config load in
Config._applied_from_default. In apply_config_override, clear those vars
before applying the custom config so only the custom file's settings take effect.

* Add config override regression test

* Make config override test setup explicit

---------

Co-authored-by: octo-patch <octo-patch@github.com>
Co-authored-by: bearsyankees <bearsyankees@gmail.com>

strix/config/config.py

@@ -2,7 +2,7 @@ import contextlib
 import json
 import os
 from pathlib import Path
-from typing import Any
+from typing import Any, ClassVar
 
 
 STRIX_API_BASE = "https://models.strix.ai/api/v1"
@@ -56,6 +56,10 @@ class Config:
     # Config file override (set via --config CLI arg)
     _config_file_override: Path | None = None
 
+    # Tracks env vars set by the initial default-config load so they can be
+    # cleared when a --config override is later applied (avoids leakage).
+    _applied_from_default: ClassVar[dict[str, str]] = {}
+
     @classmethod
     def _tracked_names(cls) -> list[str]:
         return [
@@ -151,6 +155,11 @@ class Config:
                 os.environ[var_name] = var_value
                 applied[var_name] = var_value
 
+        # Record what was applied from the default config so it can be cleared
+        # if a --config override is later provided (prevents leakage).
+        if cls._config_file_override is None and not force:
+            cls._applied_from_default = applied
+
         return applied
 
     @classmethod