Add a scope field to BotCommand so /signup, /link and /ping register globally (available in DMs — guild-scoped commands never appear there) while /announce stays guild-only. Migrate ephemeral replies to MessageFlags and make the /signup welcome message resilient to blocked DMs.
Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)
Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>
