feat: move workspaces into a first-party plugin

plugins/workspaces/frontend/index.mjs

@@ -0,0 +1,34 @@
+/**
+ * Frontend half of the workspaces plugin.
+ *
+ * WorkspacesPanel is a left-sidebar railView panel (opened from the rail,
+ * not a Tab), the same shape as FleetsPanel/HostsPanel/CredentialsPanel.
+ * None of those go through registerRailItem/registerTabComponent either --
+ * per src/ui/shell/pluginLoader.ts's own comment, a rail-view panel like
+ * this is rendered directly by AppShell.tsx rather than registered at
+ * runtime. WorkspacesPanel is reached today through a direct relative
+ * import in AppShell.tsx (../../plugins/workspaces/frontend/WorkspacesPanel),
+ * not through register() below.
+ *
+ * register()/unregister() still follow the same shape every other plugin's
+ * frontend entry does, so this is ready the moment a real frontend plugin
+ * loader exists -- see plugins/ssh-terminal/frontend/index.mjs and
+ * src/ui/tests/sidebar/plugin-extension-seam.test.tsx, which says outright
+ * "there is no plugin loader yet".
+ */
+
+export const id = "workspaces";
+export const railViewId = "workspaces";
+
+export async function register({ registerRailItem, icons }) {
+  registerRailItem?.({
+    id: railViewId,
+    icon: icons.LayoutTemplate,
+    labelKey: "nav.workspaces",
+    separatorAfter: true,
+  });
+}
+
+export async function unregister({ unregisterRailItem }) {
+  unregisterRailItem?.(railViewId);
+}