refactor(runtime): add run DDD boundary skeleton

backend/packages/harness/deerflow/runtime/runs/execution/scheduler.py

@@ -0,0 +1,25 @@
+"""Run execution scheduler contract."""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+from typing import Protocol
+
+from ..domain import RunId
+
+
+@dataclass(frozen=True)
+class RunExecutionHandle:
+    run_id: RunId
+
+
+class RunExecutionScheduler(Protocol):
+    """Starts background execution for an accepted run."""
+
+    async def start(self, run_id: RunId) -> RunExecutionHandle: ...
+
+
+__all__ = [
+    "RunExecutionHandle",
+    "RunExecutionScheduler",
+]