Feat: Let system variables appear in operator prompts #3221 (#8154)

### What problem does this PR solve?
Feat: Let system variables appear in operator prompts #3221

### Type of change


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

web/src/pages/agent/constant.tsx

@@ -31,7 +31,11 @@ import {
   initialKeywordsSimilarityWeightValue,
   initialSimilarityThresholdValue,
 } from '@/components/similarity-slider';
-import { CodeTemplateStrMap, ProgrammingLanguage } from '@/constants/agent';
+import {
+  AgentGlobals,
+  CodeTemplateStrMap,
+  ProgrammingLanguage,
+} from '@/constants/agent';
 
 export enum AgentDialogueMode {
   Conversational = 'conversational',
@@ -52,6 +56,11 @@ export enum Channel {
   News = 'news',
 }
 
+export enum PromptRole {
+  User = 'user',
+  Assistant = 'assistant',
+}
+
 import {
   BranchesOutlined,
   DatabaseOutlined,
@@ -707,7 +716,7 @@ export const initialWaitingDialogueValues = {};
 export const initialAgentValues = {
   ...initialLlmBaseValues,
   sys_prompt: ``,
-  prompts: [],
+  prompts: [{ role: PromptRole.User, content: `{${AgentGlobals.SysQuery}}` }],
   message_history_window_size: 12,
   tools: [],
   outputs: {