feat: add OpenAI Codex CLI support (AGENTS.md)

[troylar]

Summary

Add support for OpenAI Codex CLI as a new AI tool integration.

Context

OpenAI's Codex CLI gained 20k+ GitHub stars within 24 hours of launch. Included with ChatGPT Plus/Pro/Business. Uses AGENTS.md files hierarchically for AI instructions.

Details

• Install path: AGENTS.md at project root (hierarchical — also reads from subdirectories)
• File extension: .md (Markdown)
• Override support: AGENTS.override.md for local overrides
• Detection: Codex CLI binary presence
• MCP support: Yes (can run as MCP server)
• Docs: https://developers.openai.com/codex/guides/agents-md/

Notes

Uses AGENTS.md at root rather than a dotfile directory. This is a shared standard — many tools read AGENTS.md. May want to handle this as a cross-tool target rather than Codex-specific.

Acceptance Criteria

• aiconfig tools detects Codex CLI
• aiconfig install can target Codex (--ide codex)
• Instructions installed to AGENTS.md
• Unit tests with full coverage