feat: add V2 runtime hooks and compact stage prompts

[ZHOUKAILIAN]

Summary

• Add V2 lifecycle hooks with audit events, metrics, token usage capture, and runtime error blocking artifacts.
• Compact V2 stage context packets and prompts while preserving full skill routing audit in per-attempt artifacts.
• Add OpenAI executor config fallback from Codex config and fix sandbox permissions for the OpenAI SDK.

Test Plan

• npm run build
• npm test
• git diff --check
• local agt2 smoke run against group_pals test.md with CODEX_HOME override