Feature Request: show only transitive dependencies of given target tickets.

[nathan-at-least]

I'd like a feature, not sure the name, maybe FOCUS_TICKETS env which prunes all issues which aren't transitive dependencies of the tickets in FOCUS_TICKETS.

This way, when we specify a focus goal such as a specific release or feature, we can generate a DAG focused solely on that, rather than the DAG of "all potential improvements which are specified in tickets".