drop checkpoint_version from checkpoint metadata

Remove the checkpoint_version stamp from checkpoint metadata.json. It was a pure label: backend selection comes from settings topology, and nothing dispatched on the stored value. Existing checkpoints are handled by an external migration.

The checkpoint policy subsystem is untouched — its checkpoint_version field remains a checkpoint-data write guard (attach now calls the shared ensureCheckpointPolicyAllowsCheckpointData gate like explain and import), it just no longer feeds a stamp into new summaries.