Wires DAG extraction into Silo_Save and Silo_Commit commands.
After successful file upload, extracts the feature DAG and pushes
it to the server. Failures warn but never block the save.
Closes#216
Adds push_dag() and get_dag() to SiloClient:
- push_dag: PUT /api/items/{pn}/dag with nodes + edges
- get_dag: GET /api/items/{pn}/dag with optional revision filter
Closes#215