chore: drop retired orb and sun VMs

Only the zap VM remains in the fleet. Remove orb/sun from the README
architecture/config docs, the getVMClassName allowlist, and their
.timeline-vm-tag color styles.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

cmd/web-ui/static/modules/utils.js

@@ -204,7 +204,7 @@ export function getVMName(evt) {
 
 export function getVMClassName(vmName) {
   const normalized = String(vmName || 'unknown').toLowerCase();
-  return ['zap', 'orb', 'sun'].includes(normalized) ? normalized : 'unknown';
+  return ['zap'].includes(normalized) ? normalized : 'unknown';
 }
 
 export function getEventIcon(eventType) {