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/packages/harness/deerflow/agents/memory/updater.py

@@ -16,7 +16,7 @@ from deerflow.agents.memory.storage import (
     get_memory_storage,
     utc_now_iso_z,
 )
-from deerflow.config.memory_config import get_memory_config
+from deerflow.config.app_config import AppConfig
 from deerflow.models import create_chat_model
 
 logger = logging.getLogger(__name__)
@@ -265,7 +265,7 @@ class MemoryUpdater:
 
     def _get_model(self):
         """Get the model for memory updates."""
-        config = get_memory_config()
+        config = AppConfig.current().memory
         model_name = self._model_name or config.model_name
         return create_chat_model(name=model_name, thinking_enabled=False)
 
@@ -289,7 +289,7 @@ class MemoryUpdater:
         Returns:
             True if update was successful, False otherwise.
         """
-        config = get_memory_config()
+        config = AppConfig.current().memory
         if not config.enabled:
             return False
 
@@ -378,7 +378,7 @@ class MemoryUpdater:
         Returns:
             Updated memory data.
         """
-        config = get_memory_config()
+        config = AppConfig.current().memory
         now = utc_now_iso_z()
 
         # Update user sections