RFC: Add OpenAI Codex support

[tunglinwood]

Summary

Add OpenAI Codex as a first-class tool option in Ralph's loop.

Motivation

Ralph currently supports Amp and Claude Code. Many users work in Codex CLI and need an official integration path.

Proposal

• Add a CODEX.md prompt template.
• Extend ralph.sh to support --tool codex and default to codex exec --full-auto.
• Allow CODEX_CMD, CODEX_INPUT, CODEX_PROMPT_FILE, and CODEX_LAST_MESSAGE_FILE overrides.
• Detect completion via Codex --output-last-message to avoid false positives from prompt echo.
• Update README.md and AGENTS.md with setup and usage notes.

Acceptance

• ./ralph.sh --tool codex runs non-interactively without a TTY error.
• Docs explain Codex setup and configuration options.