omm export

Export a hub model to `destination` for deployment or backup: a hard link when possible, otherwise a real copy. Never a symlink, so the exported file keeps working after `omm uninstall` or on another machine. Not tracked in the registry - uninstalling the source model never touches an exported copy. Also writes a provenance/checksum manifest sidecar next to it, so `omm import` on another machine (including an air-gapped one) can restore the source repo, version, and install date instead of treating the file as an anonymous import.