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/tools/builtins/task_tool.py

@@ -92,9 +92,7 @@ async def task_tool(
     if runtime is not None:
         sandbox_state = runtime.state.get("sandbox")
         thread_data = runtime.state.get("thread_data")
-        thread_id = runtime.context.get("thread_id") if runtime.context else None
-        if thread_id is None:
-            thread_id = runtime.config.get("configurable", {}).get("thread_id")
+        thread_id = runtime.context.thread_id
 
         # Try to get parent model from configurable
         metadata = runtime.config.get("metadata", {})