Replace brew which-update

- Keep `brew which-update` as the CI-facing command because core CI now
  updates command-not-found entries from bottle metadata.
- Drop the old bulk scan, stats and commit paths because the database is
  no longer refreshed by a scheduled repository commit.
- Limit `executables_db.rb` updates to bottle JSON metadata and explicit
  formula removals.
- Regenerate command docs, completions and Sorbet RBI for the narrower
  command interface.