fix: clean up auths --help and rewrite CLI README

- Remove long_about prose that duplicated/conflicted with Commands list
- Add --help-all flag to discover hidden advanced commands
- Hide completions and --format from default help
- Change header/usage colors to blue, about line green
- Add after_help footer with discoverability hints
- Rewrite README with progressive disclosure structure
- Regenerate CLI docs via cargo xtask gen-docs

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: bordumb