feat(agent): require Chinese thinking and reports in subagent prompts (AG-4)

Subagent / vision subflow prompts (audience=agent) get a dedicated section:
report goes to the parent session, and thinking, step notes and reports stay
in Chinese even when the goal, tool results or web pages are in English.
Prompt regression test covers both the shared hard rule and the new section.

Child reports reaching the user are already humanized: the child session page
uses the same FIX-7 report-humanize path, and the parent's final report resolves
child run IDs to their goal titles; agent.spawn/await tool rows show no raw summary.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

packages/agent/src/index.ts

@@ -62,4 +62,10 @@ export {
   resolveAgentSettings,
   THINKING_LEVELS,
 } from "./settings.ts";
-export { buildSystemPrompt, interactionRules, REPORT_FORMAT, type SystemPromptInput } from "./system-prompt.ts";
+export {
+  AGENT_AUDIENCE,
+  buildSystemPrompt,
+  interactionRules,
+  REPORT_FORMAT,
+  type SystemPromptInput,
+} from "./system-prompt.ts";