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_skills_custom_router.py

@@ -6,6 +6,9 @@ from fastapi import FastAPI
 from fastapi.testclient import TestClient
 
 from app.gateway.routers import skills as skills_router
+from deerflow.config.app_config import AppConfig
+from deerflow.config.extensions_config import ExtensionsConfig
+from deerflow.config.sandbox_config import SandboxConfig
 from deerflow.skills.manager import get_skill_history_file
 from deerflow.skills.types import Skill
 
@@ -43,8 +46,7 @@ def test_custom_skills_router_lifecycle(monkeypatch, tmp_path):
         skills=SimpleNamespace(get_skills_path=lambda: skills_root, container_path="/mnt/skills"),
         skill_evolution=SimpleNamespace(enabled=True, moderation_model_name=None),
     )
-    monkeypatch.setattr("deerflow.config.get_app_config", lambda: config)
-    monkeypatch.setattr("deerflow.skills.manager.get_app_config", lambda: config)
+    monkeypatch.setattr(AppConfig, "current", staticmethod(lambda: config))
     monkeypatch.setattr("app.gateway.routers.skills.scan_skill_content", lambda *args, **kwargs: _async_scan("allow", "ok"))
     refresh_calls = []
 
@@ -93,8 +95,7 @@ def test_custom_skill_rollback_blocked_by_scanner(monkeypatch, tmp_path):
         skills=SimpleNamespace(get_skills_path=lambda: skills_root, container_path="/mnt/skills"),
         skill_evolution=SimpleNamespace(enabled=True, moderation_model_name=None),
     )
-    monkeypatch.setattr("deerflow.config.get_app_config", lambda: config)
-    monkeypatch.setattr("deerflow.skills.manager.get_app_config", lambda: config)
+    monkeypatch.setattr(AppConfig, "current", staticmethod(lambda: config))
     get_skill_history_file("demo-skill").write_text(
         '{"action":"human_edit","prev_content":' + json.dumps(original_content) + ',"new_content":' + json.dumps(edited_content) + "}\n",
         encoding="utf-8",
@@ -135,8 +136,7 @@ def test_custom_skill_delete_preserves_history_and_allows_restore(monkeypatch, t
         skills=SimpleNamespace(get_skills_path=lambda: skills_root, container_path="/mnt/skills"),
         skill_evolution=SimpleNamespace(enabled=True, moderation_model_name=None),
     )
-    monkeypatch.setattr("deerflow.config.get_app_config", lambda: config)
-    monkeypatch.setattr("deerflow.skills.manager.get_app_config", lambda: config)
+    monkeypatch.setattr(AppConfig, "current", staticmethod(lambda: config))
     monkeypatch.setattr("app.gateway.routers.skills.scan_skill_content", lambda *args, **kwargs: _async_scan("allow", "ok"))
     refresh_calls = []
 
@@ -179,9 +179,12 @@ def test_update_skill_refreshes_prompt_cache_before_return(monkeypatch, tmp_path
         refresh_calls.append("refresh")
         enabled_state["value"] = False
 
+    _app_cfg = AppConfig(sandbox=SandboxConfig(use="test"), extensions=ExtensionsConfig(mcp_servers={}, skills={}))
+
     monkeypatch.setattr("app.gateway.routers.skills.load_skills", _load_skills)
-    monkeypatch.setattr("app.gateway.routers.skills.get_extensions_config", lambda: SimpleNamespace(mcp_servers={}, skills={}))
-    monkeypatch.setattr("app.gateway.routers.skills.reload_extensions_config", lambda: None)
+    monkeypatch.setattr(AppConfig, "current", staticmethod(lambda: _app_cfg))
+    monkeypatch.setattr(AppConfig, "init", staticmethod(lambda _cfg: None))
+    monkeypatch.setattr(AppConfig, "from_file", staticmethod(lambda: _app_cfg))
     monkeypatch.setattr(skills_router.ExtensionsConfig, "resolve_config_path", staticmethod(lambda: config_path))
     monkeypatch.setattr("app.gateway.routers.skills.refresh_skills_system_prompt_cache_async", _refresh)