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/telemetry/posthog.py

@@ -1,5 +1,4 @@
 import json
-import os
 import platform
 import sys
 import urllib.request
@@ -7,6 +6,8 @@ from pathlib import Path
 from typing import TYPE_CHECKING, Any
 from uuid import uuid4
 
+from strix.config import Config
+
 
 if TYPE_CHECKING:
     from strix.telemetry.tracer import Tracer
@@ -18,7 +19,7 @@ _SESSION_ID = uuid4().hex[:16]
 
 
 def _is_enabled() -> bool:
-    return os.getenv("STRIX_TELEMETRY", "1").lower() not in ("0", "false", "no", "off")
+    return (Config.get("strix_telemetry") or "1").lower() not in ("0", "false", "no", "off")
 
 
 def _is_first_run() -> bool: