Merge refactor/config-deerflow-context into release/2.0-rc

Cherry-pick PR #2271's config refactor onto release/2.0-rc.
Used 'git merge -X theirs' to auto-resolve content conflicts in favor of
the PR's design (frozen AppConfig + explicit-parameter passing).

Limitations:
- Release-only changes that overlapped with PR's refactor in 119 files
  are NOT preserved — those files reflect PR's version. Follow-up commits
  on this branch will need to re-apply release-only modifications where
  meaningful.
- See PR #2271 for design rationale.

backend/packages/harness/deerflow/runtime/runs/manager.py

@@ -54,7 +54,7 @@ class RunManager:
         self._lock = asyncio.Lock()
         self._store = store
 
-    async def _persist_to_store(self, record: RunRecord) -> None:
+    async def _persist_to_store(self, record: RunRecord, *, follow_up_to_run_id: str | None = None) -> None:
         """Best-effort persist run record to backing store."""
         if self._store is None:
             return
@@ -68,6 +68,7 @@ class RunManager:
                 metadata=record.metadata or {},
                 kwargs=record.kwargs or {},
                 created_at=record.created_at,
+                follow_up_to_run_id=follow_up_to_run_id,
             )
         except Exception:
             logger.warning("Failed to persist run %s to store", record.run_id, exc_info=True)
@@ -89,6 +90,7 @@ class RunManager:
         metadata: dict | None = None,
         kwargs: dict | None = None,
         multitask_strategy: str = "reject",
+        follow_up_to_run_id: str | None = None,
     ) -> RunRecord:
         """Create a new pending run and register it."""
         run_id = str(uuid.uuid4())
@@ -107,7 +109,7 @@ class RunManager:
         )
         async with self._lock:
             self._runs[run_id] = record
-        await self._persist_to_store(record)
+        await self._persist_to_store(record, follow_up_to_run_id=follow_up_to_run_id)
         logger.info("Run created: run_id=%s thread_id=%s", run_id, thread_id)
         return record
 
@@ -174,6 +176,7 @@ class RunManager:
         metadata: dict | None = None,
         kwargs: dict | None = None,
         multitask_strategy: str = "reject",
+        follow_up_to_run_id: str | None = None,
     ) -> RunRecord:
         """Atomically check for inflight runs and create a new one.
 
@@ -227,7 +230,7 @@ class RunManager:
             )
             self._runs[run_id] = record
 
-        await self._persist_to_store(record)
+        await self._persist_to_store(record, follow_up_to_run_id=follow_up_to_run_id)
         logger.info("Run created: run_id=%s thread_id=%s", run_id, thread_id)
         return record