board-ensure/activate/archive took NAME positionally with no validation, so
`os-backlog board-ensure --board apprise-api` silently created a board
literally named "--board". Add a shared validate_board_name! guard, checked
right after each command shifts its name arg and before build_client, so a
flag-like name fails loudly with no Planka API call.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MuWQ8oJbFU9uPKp7dAB1Pn
jared