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/pages/AgentsPage.tsx

@@ -11,9 +11,18 @@
 // client-side into one row per agent rather than N per-agent config-PR
 // calls — exactly why the bulk endpoint exists instead of looping the
 // single-agent one.
+//
+// Owns the "+ agent" trigger too: creation used to be its own
+// `/create-agent` route + nav item, but the roster this populates is
+// the natural home for the action that populates it — a separate top-
+// level nav entry was one click of indirection for no benefit. The form
+// itself (`CreateAgentForm`) is unchanged from its page days, just
+// mounted inside a `Dialog` instead of a route.
 import { useState } from 'preact/hooks';
 import { ApiErrorPanel } from '@hive/shared/api-error-panel.js';
 import { readApiError, type ProblemDetails } from '@hive/shared/api-error.js';
+import { Button } from '../ui/button/Button.js';
+import { Dialog } from '../ui/dialog/Dialog.js';
 import { Panel } from '../ui/panel/Panel.js';
 import {
   RefreshIntervalPicker,
@@ -22,6 +31,7 @@ import {
 } from '../ui/refresh-interval/RefreshInterval.js';
 import { StatusChip } from '../ui/status-chip/StatusChip.js';
 import { Table, type TableColumn } from '../ui/table/Table.js';
+import { CreateAgentForm } from './CreateAgentForm.js';
 
 interface ConfigPrStatus {
   pr_number: number;
@@ -67,6 +77,7 @@ export function AgentsPage() {
   const [rows, setRows] = useState<AgentRow[] | null>(null);
   const [error, setError] = useState<ProblemDetails | null>(null);
   const [intervalMs, setIntervalMs] = useState<RefreshIntervalMs>(DEFAULT_INTERVAL_MS);
+  const [createOpen, setCreateOpen] = useState(false);
 
   useRefreshInterval(intervalMs, () => {
     (async () => {
@@ -93,11 +104,21 @@ export function AgentsPage() {
     <Panel
       title="agents"
       icon="👥"
-      actions={<RefreshIntervalPicker id="agents-refresh" value={intervalMs} onChange={setIntervalMs} />}
+      actions={
+        <>
+          <Button variant="primary" onClick={() => setCreateOpen(true)}>
+            + agent
+          </Button>
+          <RefreshIntervalPicker id="agents-refresh" value={intervalMs} onChange={setIntervalMs} />
+        </>
+      }
     >
       {error ? <ApiErrorPanel context="failed to load the agent roster" problem={error} /> : null}
       {!error && rows === null ? <p>loading…</p> : null}
       {rows ? <Table columns={COLUMNS} rows={rows} rowKey={(a) => a.name} /> : null}
+      <Dialog open={createOpen} onClose={() => setCreateOpen(false)} label="create agent">
+        <CreateAgentForm />
+      </Dialog>
     </Panel>
   );
 }