swarm-ui: minimal agent creation page

Adds /agents: a name field that POSTs to swarm-controller's
POST /api/agents (from the CreateIdentity work), shows the queued
job node id, and links to /jobs to watch it settle. Scope matches
the issue exactly - no forge/deploy options, those aren't wired
server-side yet.

frontend/packages/swarm-ui/src/shell/Shell.tsx

@@ -19,6 +19,7 @@ import './Shell.css';
 
 const NAV_ITEMS: { href: string; label: string }[] = [
   { href: '/', label: 'overview' },
+  { href: '/agents', label: 'agents' },
   { href: '/jobs', label: 'jobs' },
   { href: '/components', label: 'components' },
 ];