[Adapter] OpenAI Codex CLI

[shreyas-lyzr]

Summary

Add an adapter for OpenAI's Codex CLI, enabling export, run, and import support.

Details

Codex CLI is OpenAI's open-source coding agent that runs in the terminal. It uses:

• AGENTS.md at the project root for custom instructions
• codex.json or ~/.codex/config.json for model/provider configuration
• Supports multiple providers (OpenAI, Azure, custom endpoints)

A gitagent adapter would allow users to:

• Export: Convert a gitagent agent definition to Codex CLI format (AGENTS.md + config)
• Run: Execute a gitagent-defined agent using the Codex CLI runtime
• Import: Convert an existing Codex CLI project to gitagent format

Acceptance Criteria

• gitagent export --format codex generates valid AGENTS.md and codex.json
• gitagent run ./agent --adapter codex executes the agent via codex CLI
• gitagent import --from codex <path> reads AGENTS.md and produces agent.yaml + SOUL.md
• Documentation added to README adapter table
• Mapping document: what gitagent fields map, what gets dropped, what needs manual setup

Resources

• Codex CLI repo
• Existing adapters in src/adapters/ for reference (OpenCode adapter is the closest pattern)