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/checkpointer/async_provider.py

@@ -29,7 +29,7 @@ from deerflow.agents.checkpointer.provider import (
     POSTGRES_INSTALL,
     SQLITE_INSTALL,
 )
-from deerflow.config.app_config import get_app_config
+from deerflow.config.app_config import AppConfig
 from deerflow.runtime.store._sqlite_utils import ensure_sqlite_parent_dir, resolve_sqlite_conn_str
 
 logger = logging.getLogger(__name__)
@@ -94,7 +94,7 @@ async def make_checkpointer() -> AsyncIterator[Checkpointer]:
     Yields an ``InMemorySaver`` when no checkpointer is configured in *config.yaml*.
     """
 
-    config = get_app_config()
+    config = AppConfig.current()
 
     if config.checkpointer is None:
         from langgraph.checkpoint.memory import InMemorySaver