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/title_config.py

@@ -1,11 +1,13 @@
 """Configuration for automatic thread title generation."""
 
-from pydantic import BaseModel, Field
+from pydantic import BaseModel, ConfigDict, Field
 
 
 class TitleConfig(BaseModel):
     """Configuration for automatic thread title generation."""
 
+    model_config = ConfigDict(frozen=True)
+
     enabled: bool = Field(
         default=True,
         description="Whether to enable automatic title generation",
@@ -30,24 +32,3 @@ class TitleConfig(BaseModel):
         default=("Generate a concise title (max {max_words} words) for this conversation.\nUser: {user_msg}\nAssistant: {assistant_msg}\n\nReturn ONLY the title, no quotes, no explanation."),
         description="Prompt template for title generation",
     )
-
-
-# Global configuration instance
-_title_config: TitleConfig = TitleConfig()
-
-
-def get_title_config() -> TitleConfig:
-    """Get the current title configuration."""
-    return _title_config
-
-
-def set_title_config(config: TitleConfig) -> None:
-    """Set the title configuration."""
-    global _title_config
-    _title_config = config
-
-
-def load_title_config_from_dict(config_dict: dict) -> None:
-    """Load title configuration from a dictionary."""
-    global _title_config
-    _title_config = TitleConfig(**config_dict)