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/llm/memory_compressor.py

@@ -1,9 +1,10 @@
 import logging
-import os
 from typing import Any
 
 import litellm
 
+from strix.config import Config
+
 
 logger = logging.getLogger(__name__)
 
@@ -150,7 +151,7 @@ class MemoryCompressor:
         timeout: int = 600,
     ):
         self.max_images = max_images
-        self.model_name = model_name or os.getenv("STRIX_LLM", "openai/gpt-5")
+        self.model_name = model_name or Config.get("strix_llm")
         self.timeout = timeout
 
         if not self.model_name: