fix(#707): scope session GC retention (#720)

apps/gateway/src/commands/command-executor.service.ts

@@ -102,16 +102,15 @@ export class CommandExecutorService {
             success: true,
             message: 'Retry last message requested.',
           };
-        case 'gc': {
-          // Admin-only: system-wide GC sweep across all sessions
-          const result = await this.sessionGC.sweepOrphans();
+        case 'gc':
+          // Global retention requires a separate, authorized and audited job.
+          // Session cleanup is performed only through the session lifecycle.
           return {
             command: 'gc',
-            success: true,
-            message: `GC sweep complete: ${result.orphanedSessions} orphaned sessions cleaned in ${result.duration}ms.`,
+            success: false,
+            message: 'Global GC is disabled pending an authorized retention job.',
             conversationId,
           };
-        }
         case 'agent':
           return await this.handleAgent(args ?? null, conversationId, scope);
         case 'provider':