Allow gitproto.Conn and AuthMethod through ireturn

These are project-wide transport/auth interfaces, the same shape as the auth.Method entry that's already on the ireturn allowlist. Without the entries, mise run lint (which runs golangci-lint --fix) was racing: nolintlint stripped the //nolint:ireturn directives on openSource / normalizeAuth / newConn before ireturn flagged the functions, leaving lint red on every run. With the types allowed by policy, the directives become dead weight — drop them so the source documents intent once, in .golangci.yaml.