feat(app): better subagent experience (#20708)

packages/app/src/i18n/en.ts

@@ -238,6 +238,8 @@ export const dict = {
   "prompt.mode.shell": "Shell",
   "prompt.mode.normal": "Prompt",
   "prompt.mode.shell.exit": "esc to exit",
+  "session.child.promptDisabled": "Subagent sessions cannot be prompted.",
+  "session.child.backToParent": "Back to main session.",
 
   "prompt.example.1": "Fix a TODO in the codebase",
   "prompt.example.2": "What is the tech stack of this project?",