Use explicit target_ref actions for chat replies

[sleroq]

Summary

• replace username/offset-based action targeting with explicit typed actions (reply/react) that use deterministic target_ref values
• inject a Reply Target Map into the model context and resolve targets server-side from that map to avoid ambiguous history lookups
• add stable message/user markers in rendered history and keep legacy JSON output recovery for backward compatibility during rollout

Verification

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