Cross-machine sync

A tool with the sync capability moves a stored object between two machines instead of re-downloading or recomputing it. Each object has a machine-independent address — a fetched dataset by name/alias/doi, a produced artifact by cachetype[/version]/hash — so only the physical root differs per host.

• Transport is rsync over SSH; the SSH target is both transport and host identity (no remote registry).
• The remote root is resolved best-effort from the remote's own environment, then its [_STORAGE._HOST] rules, then the shared default. $repo (project-relative) is not syncable.
• Sync writes no manifest — a transferred object lands as an orphan (present, unreferenced) and is immediately usable; it is idempotent.

Normative: SCHEMA.md §Cross-machine sync.