Feat: Add the history field to the agent's system variables. #7322 (#12823)

### What problem does this PR solve?

Feat: Add the history field to the agent's system variables. #7322

### Type of change


- [x] New Feature (non-breaking change which adds functionality)

web/src/constants/agent.tsx

@@ -27,6 +27,7 @@ export enum AgentGlobals {
   SysUserId = 'sys.user_id',
   SysConversationTurns = 'sys.conversation_turns',
   SysFiles = 'sys.files',
+  SysHistory = 'sys.history',
 }
 
 export const AgentGlobalsSysQueryWithBrace = `{${AgentGlobals.SysQuery}}`;