Add delete chat flow for Discord and TUI selectors

[owenqwenstarsky]

Summary

• add a /delete command to Discord and the local TUI
• show a chat selector, then require explicit confirmation before deleting
• refresh the delete selector after each removal so multiple chats can be deleted in one session
• update active conversation state when the deleted chat was currently selected
• align shared help text and component interaction handling with the new flow

Testing

• Not run (not requested)