Add `#[completion(EntryFallback)]` syntax to enable custom
completion suggestions for unmatched entries, merging fallback
results with default completions via `Suggest::combine()`.

Update all references across docs, examples, and configuration files to
use the shorter `extras` feature name.

Archive three resolved issues by renaming files with an _ prefix and
adding
archival note banners

Mark all tasks as complete and finalize the checklist