Implement support for subagents as extensions. (#16473)

packages/core/src/index.ts

@@ -126,6 +126,7 @@ export * from './prompts/mcp-prompts.js';
 
 // Export agent definitions
 export * from './agents/types.js';
+export * from './agents/agentLoader.js';
 
 // Export specific tool logic
 export * from './tools/read-file.js';