refactor(config): eliminate global mutable state, wire DeerFlowContext into runtime

- Freeze all config models (AppConfig + 15 sub-configs) with frozen=True
- Purify from_file() — remove 9 load_*_from_dict() side-effect calls
- Replace mtime/reload/push/pop machinery with single ContextVar + init_app_config()
- Delete 10 sub-module globals and their getters/setters/loaders
- Migrate 50+ consumers from get_*_config() to get_app_config().xxx

- Expand DeerFlowContext: app_config + thread_id + agent_name (frozen dataclass)
- Wire into Gateway runtime (worker.py) and DeerFlowClient via context= parameter
- Remove sandbox_id from runtime.context — flows through ThreadState.sandbox only
- Middleware/tools access runtime.context directly via Runtime[DeerFlowContext] generic
- resolve_context() retained at server entry points for LangGraph Server fallback

backend/app/gateway/routers/models.py

@@ -1,7 +1,7 @@
 from fastapi import APIRouter, HTTPException
 from pydantic import BaseModel, Field
 
-from deerflow.config import get_app_config
+from deerflow.config.app_config import AppConfig
 
 router = APIRouter(prefix="/api", tags=["models"])
 
@@ -58,7 +58,7 @@ async def list_models() -> ModelsListResponse:
         }
         ```
     """
-    config = get_app_config()
+    config = AppConfig.current()
     models = [
         ModelResponse(
             name=model.name,
@@ -101,7 +101,7 @@ async def get_model(model_name: str) -> ModelResponse:
         }
         ```
     """
-    config = get_app_config()
+    config = AppConfig.current()
     model = config.get_model_config(model_name)
     if model is None:
         raise HTTPException(status_code=404, detail=f"Model '{model_name}' not found")