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/tests/test_security_scanner.py

@@ -2,13 +2,14 @@ from types import SimpleNamespace
 
 import pytest
 
+from deerflow.config.app_config import AppConfig
 from deerflow.skills.security_scanner import scan_skill_content
 
 
 @pytest.mark.anyio
 async def test_scan_skill_content_blocks_when_model_unavailable(monkeypatch):
     config = SimpleNamespace(skill_evolution=SimpleNamespace(moderation_model_name=None))
-    monkeypatch.setattr("deerflow.skills.security_scanner.get_app_config", lambda: config)
+    monkeypatch.setattr(AppConfig, "current", staticmethod(lambda: config))
     monkeypatch.setattr("deerflow.skills.security_scanner.create_chat_model", lambda **kwargs: (_ for _ in ()).throw(RuntimeError("boom")))
 
     result = await scan_skill_content("---\nname: demo-skill\ndescription: demo\n---\n", executable=False)