Get Pipeline Value of GitHub Issues?

Pipeline is purely a ZenHub feature.

As such, it is not exposed in the Github API Issues (or in any other GitHub API)

A ZenHub public API is in the making (issue 172).

Update: The API is available: ZenHub API

With the Github API v4, you can query the combinedContexts of a commit to retrieve a list of status contexts and check runs for the commit triggering your pipelines.