Include hidden args in completion scripts

`.hidden` arguments are public, just deprioritized, so they should be included in completion scripts. `.private` arguments, on the other hand, should stay excluded.

Start by tracing how completion scripts select public, hidden, and private arguments. Verify the existing completion-generation tests or add coverage for the distinction described in the issue. Done means `.hidden` arguments appear in completion scripts while `.private` arguments remain excluded.

Written by the indexing model from the issue text.