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/session/session-card.tsx

@@ -138,7 +138,7 @@ export function SessionCard({
 					)}
 					{session.url && (
 						<p className="session-card-url">
-							<a href={session.url} target="_blank" rel="noopener noreferrer">
+							<a href={session.url} target={`session-${session.id}`} rel="noopener noreferrer">
 								{session.url}
 							</a>
 						</p>
@@ -158,7 +158,7 @@ export function SessionCard({
 							{openHref ? (
 								<a
 									href={openHref}
-									target="_blank"
+									target={`session-${session.id}`}
 									rel="noopener noreferrer"
 									className="secondary-button mobile-primary"
 								>
@@ -215,7 +215,7 @@ export function SessionCard({
 							{openHref ? (
 								<a
 									href={openHref}
-									target="_blank"
+									target={`session-${session.id}`}
 									rel="noopener noreferrer"
 									className="secondary-button small"
 								>