refactor(runtime): introduce RunContext to reduce run_agent parameter bloat

Extract checkpointer, store, event_store, run_events_config, thread_meta_repo,
and follow_up_to_run_id into a frozen RunContext dataclass. Add get_run_context()
in deps.py to build the base context from app.state singletons. start_run() uses
dataclasses.replace() to enrich per-run fields before passing ctx to run_agent.

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

backend/packages/harness/deerflow/runtime/__init__.py

@@ -5,7 +5,7 @@ Re-exports the public API of :mod:`~deerflow.runtime.runs` and
 directly from ``deerflow.runtime``.
 """
 
-from .runs import ConflictError, DisconnectMode, RunManager, RunRecord, RunStatus, UnsupportedStrategyError, run_agent
+from .runs import ConflictError, DisconnectMode, RunContext, RunManager, RunRecord, RunStatus, UnsupportedStrategyError, run_agent
 from .serialization import serialize, serialize_channel_values, serialize_lc_object, serialize_messages_tuple
 from .store import get_store, make_store, reset_store, store_context
 from .stream_bridge import END_SENTINEL, HEARTBEAT_SENTINEL, MemoryStreamBridge, StreamBridge, StreamEvent, make_stream_bridge
@@ -14,6 +14,7 @@ __all__ = [
     # runs
     "ConflictError",
     "DisconnectMode",
+    "RunContext",
     "RunManager",
     "RunRecord",
     "RunStatus",