feat: move opkssh and warpgate into plugins (C3)

plugins/warpgate/src/frontend/index.tsx

@@ -0,0 +1,5 @@
+import type { TermixApp } from "@termix/plugin-sdk/frontend";
+
+// The host setting is drawn by core's schema form and the sign-in dialog is
+// the transports' own, so there is nothing to register.
+export function activate(_app: TermixApp): void {}