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/subagents_config.py

@@ -1,15 +1,13 @@
 """Configuration for the subagent system loaded from config.yaml."""
 
-import logging
-
-from pydantic import BaseModel, Field
-
-logger = logging.getLogger(__name__)
+from pydantic import BaseModel, ConfigDict, Field
 
 
 class SubagentOverrideConfig(BaseModel):
     """Per-agent configuration overrides."""
 
+    model_config = ConfigDict(frozen=True)
+
     timeout_seconds: int | None = Field(
         default=None,
         ge=1,
@@ -25,6 +23,8 @@ class SubagentOverrideConfig(BaseModel):
 class SubagentsAppConfig(BaseModel):
     """Configuration for the subagent system."""
 
+    model_config = ConfigDict(frozen=True)
+
     timeout_seconds: int = Field(
         default=900,
         ge=1,
@@ -62,41 +62,3 @@ class SubagentsAppConfig(BaseModel):
         if self.max_turns is not None:
             return self.max_turns
         return builtin_default
-
-
-_subagents_config: SubagentsAppConfig = SubagentsAppConfig()
-
-
-def get_subagents_app_config() -> SubagentsAppConfig:
-    """Get the current subagents configuration."""
-    return _subagents_config
-
-
-def load_subagents_config_from_dict(config_dict: dict) -> None:
-    """Load subagents configuration from a dictionary."""
-    global _subagents_config
-    _subagents_config = SubagentsAppConfig(**config_dict)
-
-    overrides_summary = {}
-    for name, override in _subagents_config.agents.items():
-        parts = []
-        if override.timeout_seconds is not None:
-            parts.append(f"timeout={override.timeout_seconds}s")
-        if override.max_turns is not None:
-            parts.append(f"max_turns={override.max_turns}")
-        if parts:
-            overrides_summary[name] = ", ".join(parts)
-
-    if overrides_summary:
-        logger.info(
-            "Subagents config loaded: default timeout=%ss, default max_turns=%s, per-agent overrides=%s",
-            _subagents_config.timeout_seconds,
-            _subagents_config.max_turns,
-            overrides_summary,
-        )
-    else:
-        logger.info(
-            "Subagents config loaded: default timeout=%ss, default max_turns=%s, no per-agent overrides",
-            _subagents_config.timeout_seconds,
-            _subagents_config.max_turns,
-        )