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/subagents/registry.py

@@ -24,9 +24,9 @@ def get_subagent_config(name: str) -> SubagentConfig | None:
         return None
 
     # Apply timeout override from config.yaml (lazy import to avoid circular deps)
-    from deerflow.config.subagents_config import get_subagents_app_config
+    from deerflow.config.app_config import AppConfig
 
-    app_config = get_subagents_app_config()
+    app_config = AppConfig.current().subagents
     effective_timeout = app_config.get_timeout_for(name)
     effective_max_turns = app_config.get_max_turns_for(name, config.max_turns)