feat: convert serial console into the serial plugin

plugins/serial/src/backend/index.ts

@@ -0,0 +1,12 @@
+import type { PluginContext } from "@termix/plugin-sdk/backend";
+import { createSerialSession } from "./session.js";
+
+export async function activate(ctx: PluginContext): Promise<void> {
+  ctx.ws.route("/console", createSerialSession(ctx));
+  ctx.log.info("Serial console mounted at /plugin-ws/serial/console");
+}
+
+export async function deactivate(): Promise<void> {
+  // The WS route and every open socket are torn down by core: see
+  // src/backend/plugins/ws.ts. Nothing here outlives activate().
+}