fix(cli): keep an internal code note out of br update --help (GH #516)

clap turns every /// paragraph on a subcommand variant into help text, so the
note about boxing UpdateArgs showed up as the long description of
br update --help. Make it a plain // comment and add a test that walks every
(sub)command's about/long-about for leaked developer notes.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>