feat: ship the ssh terminal as a first-party in-process plugin

Adds a hardcoded process:transport-owner tier so the terminal can own its
WebSocket server and ssh2 clients, which cannot cross the worker boundary.

plugins/ssh-terminal/frontend/index.mjs

@@ -0,0 +1,41 @@
+/**
+ * Frontend half of the ssh-terminal plugin.
+ *
+ * Registers the existing Terminal component through the extension seams that
+ * already exist in the shell (registerRailItem / registerTabComponent). The
+ * component itself is unchanged and still lives in src/ui/features/terminal -
+ * this only controls whether the shell knows about it.
+ *
+ * register() is called when the plugin is enabled, unregister() when it is
+ * disabled, so the tab and rail entry appear and disappear with the plugin.
+ */
+
+export const id = "ssh-terminal";
+export const tabId = "terminal";
+
+export async function register({
+  registerRailItem,
+  registerTabComponent,
+  icons,
+}) {
+  registerTabComponent(tabId, () =>
+    import("@/features/terminal/Terminal").then((m) => ({
+      default: m.Terminal,
+    })),
+  );
+
+  registerRailItem({
+    id: tabId,
+    icon: icons.SquareTerminal,
+    labelKey: "nav.terminal",
+    kind: "tab",
+  });
+}
+
+export async function unregister({
+  unregisterRailItem,
+  unregisterTabComponent,
+}) {
+  unregisterRailItem(tabId);
+  unregisterTabComponent(tabId);
+}