Orchestrate a multi-phase implementation workflow for this repository with artifact files under .ai/
// using Codex subagents instead of shell-spawned child processes. Use when the user wants one prompt to drive context gathering, planning, plan assessment, implementation, build verification, and review with persistent artifacts, clear phase handoffs, and a thin parent thread. Prefer spawn_agent/send_input/wait_agent, keep heavy pre-build work delegated when possible, and avoid pulling timed-out phases back into the main session.