Add Codex SDK engine integration

[Copilot]

Adds a Codex-backed Rig engine with persistent conversations, system instructions, cancellation, and lifecycle management.

• Engine

  • Export codexEngine() from rig/engines/codex
  • Preserve Codex threads across repair turns
  • Map Rig system messages to Codex developer instructions
  • Forward cancellation and abort active turns on close
  • Reject unsupported Rig custom tools explicitly

• Configuration

  • Accept Codex client options and nested thread options
  • Add @openai/codex-sdk as a dependency

• Usage

import { configureAgent } from "rig";
import { codexEngine } from "rig/engines/codex";

configureAgent(codexEngine({
  thread: { approvalPolicy: "never" },
}));

• Documentation
  • Document configuration, conversation behavior, and tool limitations