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/sandbox/sandbox_provider.py

@@ -1,6 +1,6 @@
 from abc import ABC, abstractmethod
 
-from deerflow.config import get_app_config
+from deerflow.config.app_config import AppConfig
 from deerflow.reflection import resolve_class
 from deerflow.sandbox.sandbox import Sandbox
 
@@ -50,7 +50,7 @@ def get_sandbox_provider(**kwargs) -> SandboxProvider:
     """
     global _default_sandbox_provider
     if _default_sandbox_provider is None:
-        config = get_app_config()
+        config = AppConfig.current()
         cls = resolve_class(config.sandbox.use, SandboxProvider)
         _default_sandbox_provider = cls(**kwargs)
     return _default_sandbox_provider