swarm-ui: move agent creation into the agents page as a dialog

Adds a generic ui/dialog/Dialog primitive (native <dialog>, no
third-party modal lib and no shadow-DOM custom element per the esbuild
gap on those) and wires a "+ agent" button into AgentsPage that opens
the existing create-agent form inside it, content unchanged from its
former life as a standalone /create-agent route/nav item.

Removes the "new agent" top-level nav entry and the /create-agent
route entirely -- creation now only reachable from the roster that
gets populated by it. CreateAgentPage.tsx/css renamed to
CreateAgentForm.tsx/css to match its new role as a mounted component
rather than a page.

Screenshot-verified the dialog open/closed states against a mock
server.

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

@@ -39,7 +39,6 @@ import './Shell.css';
 const NAV_ITEMS: { href: string; label: string; accent: string }[] = [
   { href: '/', label: 'hives', accent: 'var(--purple)' },
   { href: '/agents', label: 'agents', accent: 'var(--green)' },
-  { href: '/create-agent', label: 'new agent', accent: 'var(--cyan)' },
   { href: '/jobs', label: 'jobs', accent: 'var(--pink)' },
   { href: '/components', label: 'components', accent: 'var(--blue)' },
 ];