Move the top-level usage/command-list text out of the dispatcher into
commands/help.py as cmd_help, registered in COMMANDS so `bb help` now
works as a real command. The dispatcher's -h/--help, no-args, and
unknown-command fallbacks call cmd_help() for the text while keeping
their exit codes (0 for help/-h, 2 for bare no-args, Die for unknown).
Added `help` to NO_MIGRATION_COMMANDS so it prints without a migrated
DB, and listed it in the command summary. Full unit suite green (2243);
dispatch + migration-gate tests pass, `bb help` verified.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
