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/acp_config.py

@@ -1,16 +1,13 @@
 """ACP (Agent Client Protocol) agent configuration loaded from config.yaml."""
 
-import logging
-from collections.abc import Mapping
-
-from pydantic import BaseModel, Field
-
-logger = logging.getLogger(__name__)
+from pydantic import BaseModel, ConfigDict, Field
 
 
 class ACPAgentConfig(BaseModel):
     """Configuration for a single ACP-compatible agent."""
 
+    model_config = ConfigDict(frozen=True)
+
     command: str = Field(description="Command to launch the ACP agent subprocess")
     args: list[str] = Field(default_factory=list, description="Additional command arguments")
     env: dict[str, str] = Field(default_factory=dict, description="Environment variables to inject into the agent subprocess. Values starting with $ are resolved from host environment variables.")
@@ -24,28 +21,3 @@ class ACPAgentConfig(BaseModel):
             "are denied — the agent must be configured to operate without requesting permissions."
         ),
     )
-
-
-_acp_agents: dict[str, ACPAgentConfig] = {}
-
-
-def get_acp_agents() -> dict[str, ACPAgentConfig]:
-    """Get the currently configured ACP agents.
-
-    Returns:
-        Mapping of agent name -> ACPAgentConfig.  Empty dict if no ACP agents are configured.
-    """
-    return _acp_agents
-
-
-def load_acp_config_from_dict(config_dict: Mapping[str, Mapping[str, object]] | None) -> None:
-    """Load ACP agent configuration from a dictionary (typically from config.yaml).
-
-    Args:
-        config_dict: Mapping of agent name -> config fields.
-    """
-    global _acp_agents
-    if config_dict is None:
-        config_dict = {}
-    _acp_agents = {name: ACPAgentConfig(**cfg) for name, cfg in config_dict.items()}
-    logger.info("ACP config loaded: %d agent(s): %s", len(_acp_agents), list(_acp_agents.keys()))