feat: implement session deletion protection and enhance chat history management

electron/gateway/manager.ts

@@ -798,6 +798,10 @@ class GatewayManager {
       }
       case 'session.delete': {
         const request = params as GatewayRpcParams['session.delete'];
+        if (normalizeAgentSessionKey(request.sessionKey).endsWith(':main')) {
+          return { success: false };
+        }
+
         await this.rpcGateway('sessions.delete', {
           key: normalizeAgentSessionKey(request.sessionKey),
           deleteTranscript: true,