#861: Reject ambiguous GitHub identifiers

[gemshrine]

Fbe.who and Fbe.issue read the first value from identifier properties. When a fact contains multiple who, repository, or issue values, the helpers silently discard the rest and can format a different user or issue than the fact represents.

The helpers now raise Fbe::Error before making an API lookup when an identifier property has multiple values. Fbe.who applies the same validation when callers provide a custom property name.

Regression tests cover multiple user IDs, a custom user ID property, multiple repository IDs, and multiple issue numbers. The repository suite runs these tests with bundle exec rake test.

Fixes #861