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/app/gateway/deps.py

@@ -14,7 +14,7 @@ from contextlib import AsyncExitStack, asynccontextmanager
 
 from fastapi import FastAPI, HTTPException, Request
 
-from deerflow.runtime import RunManager
+from deerflow.runtime import RunContext, RunManager
 
 
 @asynccontextmanager
@@ -109,6 +109,25 @@ def get_thread_meta_repo(request: Request):
     return getattr(request.app.state, "thread_meta_repo", None)
 
 
+def get_run_context(request: Request) -> RunContext:
+    """Build a :class:`RunContext` from ``app.state`` singletons.
+
+    Returns a *base* context with infrastructure dependencies.  Callers that
+    need per-run fields (e.g. ``follow_up_to_run_id``) should use
+    ``dataclasses.replace(ctx, follow_up_to_run_id=...)`` before passing it
+    to :func:`run_agent`.
+    """
+    from deerflow.config import get_app_config
+
+    return RunContext(
+        checkpointer=get_checkpointer(request),
+        store=get_store(request),
+        event_store=get_run_event_store(request),
+        run_events_config=getattr(get_app_config(), "run_events", None),
+        thread_meta_repo=get_thread_meta_repo(request),
+    )
+
+
 async def get_current_user(request: Request) -> str | None:
     """Extract user identity from request.