feat(persistence): add RunEventStore ABC + MemoryRunEventStore

Phase 2-A prerequisite for event storage: adds the unified run event
stream interface (RunEventStore) with an in-memory implementation,
RunEventsConfig, gateway integration, and comprehensive tests (27 cases).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/packages/harness/deerflow/config/app_config.py

@@ -14,6 +14,7 @@ from deerflow.config.extensions_config import ExtensionsConfig
 from deerflow.config.guardrails_config import load_guardrails_config_from_dict
 from deerflow.config.memory_config import load_memory_config_from_dict
 from deerflow.config.model_config import ModelConfig
+from deerflow.config.run_events_config import RunEventsConfig
 from deerflow.config.sandbox_config import SandboxConfig
 from deerflow.config.skills_config import SkillsConfig
 from deerflow.config.stream_bridge_config import StreamBridgeConfig, load_stream_bridge_config_from_dict
@@ -43,6 +44,7 @@ class AppConfig(BaseModel):
     tool_search: ToolSearchConfig = Field(default_factory=ToolSearchConfig, description="Tool search / deferred loading configuration")
     model_config = ConfigDict(extra="allow", frozen=False)
     database: DatabaseConfig = Field(default_factory=DatabaseConfig, description="Unified database backend configuration")
+    run_events: RunEventsConfig = Field(default_factory=RunEventsConfig, description="Run event storage configuration")
     checkpointer: CheckpointerConfig | None = Field(default=None, description="Checkpointer configuration")
     stream_bridge: StreamBridgeConfig | None = Field(default=None, description="Stream bridge configuration")