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/checkpointer_config.py

@@ -2,7 +2,7 @@
 
 from typing import Literal
 
-from pydantic import BaseModel, Field
+from pydantic import BaseModel, ConfigDict, Field
 
 CheckpointerType = Literal["memory", "sqlite", "postgres"]
 
@@ -10,6 +10,8 @@ CheckpointerType = Literal["memory", "sqlite", "postgres"]
 class CheckpointerConfig(BaseModel):
     """Configuration for LangGraph state persistence checkpointer."""
 
+    model_config = ConfigDict(frozen=True)
+
     type: CheckpointerType = Field(
         description="Checkpointer backend type. "
         "'memory' is in-process only (lost on restart). "
@@ -23,24 +25,3 @@ class CheckpointerConfig(BaseModel):
         "For sqlite, use a file path like '.deer-flow/checkpoints.db' or ':memory:' for in-memory. "
         "For postgres, use a DSN like 'postgresql://user:pass@localhost:5432/db'.",
     )
-
-
-# Global configuration instance — None means no checkpointer is configured.
-_checkpointer_config: CheckpointerConfig | None = None
-
-
-def get_checkpointer_config() -> CheckpointerConfig | None:
-    """Get the current checkpointer configuration, or None if not configured."""
-    return _checkpointer_config
-
-
-def set_checkpointer_config(config: CheckpointerConfig | None) -> None:
-    """Set the checkpointer configuration."""
-    global _checkpointer_config
-    _checkpointer_config = config
-
-
-def load_checkpointer_config_from_dict(config_dict: dict) -> None:
-    """Load checkpointer configuration from a dictionary."""
-    global _checkpointer_config
-    _checkpointer_config = CheckpointerConfig(**config_dict)