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/memory_config.py

@@ -1,11 +1,13 @@
 """Configuration for memory mechanism."""
 
-from pydantic import BaseModel, Field
+from pydantic import BaseModel, ConfigDict, Field
 
 
 class MemoryConfig(BaseModel):
     """Configuration for global memory mechanism."""
 
+    model_config = ConfigDict(frozen=True)
+
     enabled: bool = Field(
         default=True,
         description="Whether to enable memory mechanism",
@@ -59,24 +61,3 @@ class MemoryConfig(BaseModel):
         le=8000,
         description="Maximum tokens to use for memory injection",
     )
-
-
-# Global configuration instance
-_memory_config: MemoryConfig = MemoryConfig()
-
-
-def get_memory_config() -> MemoryConfig:
-    """Get the current memory configuration."""
-    return _memory_config
-
-
-def set_memory_config(config: MemoryConfig) -> None:
-    """Set the memory configuration."""
-    global _memory_config
-    _memory_config = config
-
-
-def load_memory_config_from_dict(config_dict: dict) -> None:
-    """Load memory configuration from a dictionary."""
-    global _memory_config
-    _memory_config = MemoryConfig(**config_dict)