repo create: guard repoCreateOutput against a nil repo

repoCreateOutput dereferenced its *Repo argument (via *r in the remote synthesis) without a nil check. The sole caller only passes a repo after a nil-error create, so this defends the contract rather than a live path — but a nil pointer would slip past json.Marshal (which encodes a nil pointer as "null" without error) and panic later on the dereference. Return an error up front instead.