refactor(persistence): unify SQLite to single deerflow.db and move checkpointer to runtime

Merge checkpoints.db and app.db into a single deerflow.db file (WAL mode
handles concurrent access safely). Move checkpointer module from
agents/checkpointer to runtime/checkpointer to better reflect its role
as a runtime infrastructure concern.

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

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

@@ -1,4 +1,3 @@
-from .checkpointer import get_checkpointer, make_checkpointer, reset_checkpointer
 from .factory import create_deerflow_agent
 from .features import Next, Prev, RuntimeFeatures
 from .lead_agent import make_lead_agent
@@ -18,7 +17,4 @@ __all__ = [
     "make_lead_agent",
     "SandboxState",
     "ThreadState",
-    "get_checkpointer",
-    "reset_checkpointer",
-    "make_checkpointer",
 ]