feat: add centralized Config class with auto-save to ~/.strix/cli-config.json

- Add Config class with all env var defaults in one place
- Auto-load saved config on startup (env vars take precedence)
- Auto-save config after successful LLM warm-up
- Replace scattered os.getenv() calls with Config.get()

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

strix/config/__init__.py

@@ -0,0 +1,12 @@
+from strix.config.config import (
+    Config,
+    apply_saved_config,
+    save_current_config,
+)
+
+
+__all__ = [
+    "Config",
+    "apply_saved_config",
+    "save_current_config",
+]