feat: reuse existing tabs when opening sessions instead of always creating new ones

Use named window targets (session-{id}, instance-{id}) instead of _blank:
- use-instance-actions.ts: window.open(..., 'session-{id}') for programmatic opens
- session-card.tsx: <a target='session-{id}'> for direct link opens (mobile + desktop)
- workspace-instance-chips.tsx: <a target='instance-{id}'> for chip links
- instance-list.tsx: <a target='instance-{id}'> for instance open links

Browser behavior: if a tab with that target name exists, it navigates/focuses
that tab instead of opening a new one.

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

apps/web/src/components/features/tool/instance-list.tsx

@@ -282,7 +282,7 @@ export const InstanceList = ({
 														? instance.url
 														: `${API_BASE_URL}${instance.url}`
 												}
-												target="_blank"
+												target={`instance-${instance.id}`}
 												rel="noopener noreferrer"
 												className="secondary-button small"
 											>