Client.SendMessage(title, raw, targets) creates a private-message
topic by posting to /posts.json with target_usernames /
target_group_names / target_emails (comma-joined on the wire, as
Discourse expects, never JSON arrays) instead of a category. The
result reuses the topic ids type, so URL() renders the PM link.
Local validation requires title, body and at least one target;
undeliverable recipients surface as the typed ErrForbidden class.
Fake-server tests assert the wire shape and the recorded recipients.
Part of Redmine 507 (Discourse Go client): PM delivery is how the
495 pipeline pings humans that a briefing has landed.
