adds tg repo fork --knot <host> for cross-knot forks. the fork remains a formal fork: it records source provenance as the source knot URL plus repo DID, while creating the destination repository and record on the selected knot.
validated live by formally forking callie.on-her.computer/coral from knot.bas.sh onto knot1.tangled.sh as niri.pet/coral.
verification: nix develop -c go test ./....