Refactor AI prompt helpers and history selection

[sleroq]

Summary

• extract target ref mapping/annotation and language protocol into standalone pure modules used by Telegram AI handler
• refactor history building with a shared selectHistoryCandidates pipeline used by both chat history and notes history generation
• add focused tests for new helper modules and history candidate selection behavior

Verification

• deno check --allow-import main.ts
• deno lint
• AI_TOKEN="test" deno test -A