Work with iMessage conversations synced to ~/Documents/alto-index/messages/.
If $ARGUMENTS is provided, search for that contact name or text across all conversations.
If no arguments, list all conversations:
- show contact/group name, message count, and date range (from frontmatter)
- sort by most recent (last_message field)
- indicate which are group chats
If the messages folder doesn't exist, tell the user to enable iMessage sync in alto.index.