Document replicate API and stable-surface JSON rename

Adds a CHANGELOG describing the replicate mode, the renamed execution.mode -> execution.transfer_mode field on the stable SyncResult surface, and the new top-level operation_mode on syncer.Result. Updates the embedding guide to list Replicate alongside Probe/Plan/Sync and to name the new Execution fields (OperationMode, TransferMode) in the result-handling and retry-guidance sections.

The rename is breaking for external consumers parsing execution.mode; there is no compatibility alias. This is flagged under "Changed (stable API, breaking)" in the CHANGELOG.