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/config/__init__.py

@@ -1,6 +1,6 @@
-from .app_config import get_app_config
-from .extensions_config import ExtensionsConfig, get_extensions_config
-from .memory_config import MemoryConfig, get_memory_config
+from .app_config import AppConfig
+from .extensions_config import ExtensionsConfig
+from .memory_config import MemoryConfig
 from .paths import Paths, get_paths
 from .skill_evolution_config import SkillEvolutionConfig
 from .skills_config import SkillsConfig
@@ -13,18 +13,16 @@ from .tracing_config import (
 )
 
 __all__ = [
-    "get_app_config",
-    "SkillEvolutionConfig",
-    "Paths",
-    "get_paths",
-    "SkillsConfig",
+    "AppConfig",
     "ExtensionsConfig",
-    "get_extensions_config",
     "MemoryConfig",
-    "get_memory_config",
-    "get_tracing_config",
-    "get_explicitly_enabled_tracing_providers",
+    "Paths",
+    "SkillEvolutionConfig",
+    "SkillsConfig",
     "get_enabled_tracing_providers",
+    "get_explicitly_enabled_tracing_providers",
+    "get_paths",
+    "get_tracing_config",
     "is_tracing_enabled",
     "validate_enabled_tracing_providers",
 ]