fix: remove rel=noopener from all session/instance links to enable tab reuse

rel=noopener forces a fresh browsing context, breaking target name matching
and window reference tracking across browsers.

Changes:
- use-instance-actions.ts: track opened tabs in Map, call .focus() on existing
- session-card.tsx: all Open buttons now go through handleOpen callback
- app-shell.tsx: sidebar session links use target='session-{id}' + noreferrer
- workspace-instance-chips.tsx: remove noopener from chip links
- instance-list.tsx: remove noopener from Open links
- WorkspaceDetailPage.tsx: use named target + noreferrer

Quality gates: tsc --noEmit pass, npm run build pass, 80/80 tests pass

apps/web/src/pages/WorkspaceDetailPage.tsx

@@ -343,8 +343,8 @@ function ToolsTab({ workspace }: { workspace: Workspace }) {
 								{instance.url && (
 									<a
 										href={instance.url}
-										target="_blank"
-										rel="noopener noreferrer"
+										target={`instance-${instance.id}`}
+										rel="noreferrer"
 									>
 										Open
 									</a>