Maity: Board

cli: maity say cnv-… without --chat hints at --chat

tsk-w0zws1c72zm3 done fix an agent

From the first-day feedback of marketing-head#776r (seq 19939): 'maity say cnv-… -' fails with a message about --type. When the address starts with 'cnv-', the usage error should say: group chats take --chat <id>. Small, with a test.