Navigate Rose-internal links in the app, not a new tab

Every markdown link in an assistant message rendered with target="_blank",
including relative ones. So the K&S matter link the assistant now returns after
every write opened a new tab and reloaded the entire app, dropping the user out
of the shell - the precise thing /workspace was built to avoid.

Relative hrefs now go through next/link: same tab, client-side, sidebar intact.
External links keep _blank and noopener.

Checked while investigating a reported truncation of that link: the stored
messages are complete, closing bracket and all, and nothing in the renderer
clamps. The truncation was in copying out of the browser.