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/ui/dialog/Dialog.tsx

@@ -0,0 +1,76 @@
+// <Dialog> — a generic modal overlay, native `<dialog>` element rather
+// than a hand-rolled focus-trap + backdrop: `showModal()` gives focus
+// trapping, Escape-to-close, and a real `::backdrop` for free, and
+// `@hive/shared`'s own shadow-DOM `hive-dialog` custom element isn't an
+// option here — swarm-ui's esbuild config can't consume those custom
+// elements at all (a separate, already-tracked gap).
+// No opinion on content beyond a close button; the caller's own markup
+// supplies whatever heading/layout it needs (its first real caller,
+// the create-agent form, already has an established two-panel layout
+// from its former life as a standalone route — reusing that unchanged
+// is simpler and lower-risk than re-deriving a modal-specific layout
+// for content mara has already reviewed).
+import { useEffect, useRef } from 'preact/hooks';
+import type { ComponentChildren } from 'preact';
+import './Dialog.css';
+
+export function Dialog({
+  open,
+  onClose,
+  label,
+  children,
+}: {
+  open: boolean;
+  onClose: () => void;
+  // `aria-label` only, no visible title row — see file-top comment on
+  // why content owns its own heading.
+  label: string;
+  children: ComponentChildren;
+}) {
+  const ref = useRef<HTMLDialogElement>(null);
+
+  // Drives the native open/closed state from the `open` prop rather
+  // than mounting/unmounting the element — `showModal()`/`close()` are
+  // imperative, there's no declarative `<dialog open>` equivalent that
+  // also gets you the backdrop + focus trap.
+  useEffect(() => {
+    const el = ref.current;
+    if (!el) return;
+    if (open && !el.open) el.showModal();
+    else if (!open && el.open) el.close();
+  }, [open]);
+
+  // The dialog's own `close` event fires on Escape (and would fire on
+  // a native <form method="dialog"> submit, unused here) — syncing it
+  // back to the caller's state keeps `open` truthful even when nothing
+  // in this component's own JS drove the close.
+  useEffect(() => {
+    const el = ref.current;
+    if (!el) return;
+    function handleClose() {
+      onClose();
+    }
+    el.addEventListener('close', handleClose);
+    return () => el.removeEventListener('close', handleClose);
+  }, [onClose]);
+
+  return (
+    <dialog
+      ref={ref}
+      class="ui-dialog"
+      aria-label={label}
+      // A click lands on the `<dialog>` element itself (not any child)
+      // exactly when it's outside the rendered content box — inside
+      // the box, the click target is always some descendant. Standard
+      // "click the backdrop to close" trick for native `<dialog>`.
+      onClick={(e) => {
+        if (e.target === ref.current) onClose();
+      }}
+    >
+      <button type="button" class="ui-dialog-close" aria-label="close" onClick={onClose}>
+        ✕
+      </button>
+      <div class="ui-dialog-body">{children}</div>
+    </dialog>
+  );
+}