Advanced Codex CLI Playbook: Layered AGENTS.md + config.toml + Research-to-Ship Workflow

Upgrades OpenAI Codex CLI from 'vibe coding' into a controllable engineering agent: layered AGENTS.md, approval and sandbox settings in .codex/config.toml, and Starlark command gating, paired with a five-stage Research → Plan → Execute → Review → Ship workflow.