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.

src/backend/plugins/protocol.ts

@@ -31,7 +31,7 @@ export type PluginResponse = PluginResponseOk | PluginResponseErr;
 
 /** Main thread -> worker, pushed rather than requested. */
 export interface PluginPush {
-  kind: "event" | "http" | "timer";
+  kind: "event" | "http" | "timer" | "deactivate";
   /** Event topic, route key, or timer id depending on kind. */
   key: string;
   payload: unknown;