feat: implement server pairing confirmation flow

plugin/core/persistence.ts

@@ -110,6 +110,9 @@ export interface ClientSession {
   /** WebSocket connection instance */
   readonly socket: unknown; // Will be typed as WebSocket when implementing transport
 
+  /** Public key presented during hello, before pairing completes */
+  publicKey?: string;
+
   /** Whether the client is currently authenticated */
   isAuthenticated: boolean;