For tools whose reach is the point: a Quick Look preview covers any
format with a plugin installed, so filtering by a known extension list
would silently drop the very files the caller wanted looked at and then
report success over what is left. Same shape for a duplicate scan or a
secret sweep.

gather(_:matching:) keeps its signature and now delegates to a private
form taking an optional set; nil means take everything.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SNXBmZ6Ad8bc8GhCD1exWx