fix(doctor): offer a git command that works for the remote's actual state

The fail-closed note told every unfetchable checkpoint_push_remote to run git remote set-url <name> <url>, which fails with "No such remote" when the name is missing from this clone, the common case. Offer git remote add for a missing remote and keep set-url for a pushurl-only one.

The name can come from the committed settings file, so it is printed inside a command only when it is a plain remote name; anything else gets the remedy without a command.

Addresses trail 1303 findings 01M6NKJHYASBP6SAC4Q3NTCX8B and 01M6NKJK5S4HJ3K32PGBTJGSXG.