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/runtime/__init__.py

@@ -1,4 +1,4 @@
-import os
+from strix.config import Config
 
 from .runtime import AbstractRuntime
 
@@ -13,7 +13,7 @@ class SandboxInitializationError(Exception):
 
 
 def get_runtime() -> AbstractRuntime:
-    runtime_backend = os.getenv("STRIX_RUNTIME_BACKEND", "docker")
+    runtime_backend = Config.get("strix_runtime_backend")
 
     if runtime_backend == "docker":
         from .docker_runtime import DockerRuntime