feat: ship host metrics as a first-party in-process plugin

plugins/host-metrics/frontend/index.mjs

@@ -0,0 +1,49 @@
+/**
+ * Frontend half of the host-metrics plugin.
+ *
+ * Registers the Host Metrics tab through the existing extension seams
+ * (registerRailItem / registerTabComponent), the same way ssh-terminal's and
+ * docker's frontend do.
+ *
+ * HostEditorStatsTab itself stays in src/ui/sidebar/HostEditorStatsTab.tsx
+ * rather than moving into this plugin: it takes the same {form, setField}
+ * pair every other host editor tab does, and those types (HostEditorForm,
+ * the setField signature) are core types HostEditor.tsx owns -- the same
+ * reason docker leaves HostDockerTab in core.
+ *
+ * register() is called when the plugin is enabled, unregister() when it is
+ * disabled. As of this change neither is actually invoked anywhere yet, the
+ * same as ssh-terminal's and docker's frontend entries -- there is no
+ * frontend plugin loader yet. This file follows the same shape so it is
+ * ready the moment that loader exists.
+ */
+
+export const id = "host-metrics";
+export const tabId = "host-metrics";
+
+export async function register({
+  registerRailItem,
+  registerTabComponent,
+  icons,
+}) {
+  registerTabComponent(tabId, () =>
+    import("./HostMetricsTab.tsx").then((m) => ({
+      default: m.HostMetricsTab,
+    })),
+  );
+
+  registerRailItem({
+    id: tabId,
+    icon: icons.Activity,
+    labelKey: "nav.hostMetrics",
+    kind: "tab",
+  });
+}
+
+export async function unregister({
+  unregisterRailItem,
+  unregisterTabComponent,
+}) {
+  unregisterRailItem(tabId);
+  unregisterTabComponent(tabId);
+}