Implement subagent prompt caching: includeMessageHistory now also includes system prompt

Author: jahooma

backend/src/__tests__/cost-aggregation.integration.test.ts

@@ -48,7 +48,7 @@ const mockFileContext: ProjectFileContext = {
       inputSchema: {},
       spawnerPrompt: '',
       model: 'gpt-4o-mini',
-      includeMessageHistory: true,
+      includeMessageHistory: false,
       toolNames: ['spawn_agents'],
       spawnableAgents: ['editor'],
       systemPrompt: 'Base agent system prompt',
@@ -65,7 +65,7 @@ const mockFileContext: ProjectFileContext = {
       includeMessageHistory: true,
       toolNames: ['write_file'],
       spawnableAgents: [],
-      systemPrompt: 'Editor agent system prompt',
+      systemPrompt: '',
       instructionsPrompt: 'Editor agent instructions',
       stepPrompt: 'Editor agent step prompt',
     },
@@ -139,7 +139,7 @@ describe('Cost Aggregation Integration Tests', () => {
         inputSchema: {},
         spawnerPrompt: '',
         model: 'gpt-4o-mini',
-        includeMessageHistory: true,
+        includeMessageHistory: false,
         mcpServers: {},
         toolNames: ['spawn_agents'],
         spawnableAgents: ['editor'],
@@ -158,7 +158,7 @@ describe('Cost Aggregation Integration Tests', () => {
         mcpServers: {},
         toolNames: ['write_file'],
         spawnableAgents: [],
-        systemPrompt: 'Editor agent system prompt',
+        systemPrompt: '',
         instructionsPrompt: 'Editor agent instructions',
         stepPrompt: 'Editor agent step prompt',
       } satisfies AgentTemplate,