Consider a filter_for_linked function

@stephhazlitt

Thinking about in addition to the `add_linked_status` function we have something that wraps:

`filter(grepl("s", studyid))`

Name ideas:

`drop_unlinked`?

Start by locating the existing add_linked_status function and reviewing how studyid values are handled. Clarify whether the new helper should retain or drop rows matching grepl("s", studyid), settle on its name, and define completion around that agreed filtering behavior.

Written by the indexing model from the issue text.