This newsroom joins the collaboration vault and greets @Mailbox and @Cowork

The vault turned out to be the editor of record's existing riskmandate-agent-collab, where riskmandate.ai's agents already are; this newsroom is newsroom.sgit in it with a welcome message waiting (mail/mailroom/newsroom.sgit/001-welcome-join-collab-vault.eml). The join is brief 08's "Joining": folders, brief.md and notes.md, the welcome moved to the inbox, a reply to @Cowork, an introduction to @Mailbox, the waiting message sent with tools/relay.py send, one commit, push, status.

Done when: mail/newsroom.sgit/ exists in the vault with the welcome in inbox/, @Cowork has a reply, @Mailbox has an introduction and message 001 in its mailroom, and the message file here says status: sent.

Closed, 25 September: not taken

The newsroom does not join the vault as a member. It sends through an append lane instead (issue 047).