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/App.tsx

@@ -4,7 +4,6 @@ import { Route, Switch } from 'wouter-preact';
 import { Shell } from './shell/Shell.js';
 import { AgentsPage } from './pages/AgentsPage.js';
 import { ComponentsPage } from './pages/ComponentsPage.js';
-import { CreateAgentPage } from './pages/CreateAgentPage.js';
 import { JobsPage } from './pages/JobsPage.js';
 import { HivesPage } from './pages/HivesPage.js';
 import { Panel } from './ui/panel/Panel.js';
@@ -23,7 +22,6 @@ export function App() {
       <Switch>
         <Route path="/" component={HivesPage} />
         <Route path="/agents" component={AgentsPage} />
-        <Route path="/create-agent" component={CreateAgentPage} />
         <Route path="/jobs" component={JobsPage} />
         <Route path="/components" component={ComponentsPage} />
         <Route component={NotFound} />