officer is a single-page app, so a same-tab navigation to an external url
unloads it — tap a link in a message and you come back to a cold boot with the
scroll position and anything mid-flight gone.
`a` was simply absent from MD_COMPONENTS, so every link rendered as a bare
anchor. all four ReactMarkdown call sites in MessageBubble share that object,
so one override covers assistant messages, user messages, tool output and the
collapsed blocks.
noopener noreferrer with it: a bare target="_blank" hands the opened page a
live window.opener back into the app. target is set AFTER the prop spread so
raw html in a message cannot override it with _self.