Add Codex CLI runner support

[xxddccaa]

Summary

• add --tool codex support to ralph.sh
• add a Codex-specific CODEX.md prompt template
• document Codex setup, usage, and key files

Testing

• bash -n ralph.sh
• invalid tool validation smoke test
• fake codex runner smoke test
• real Codex CLI isolated smoke test with completed prd.json

[Keesan12]

Nice addition. One thing we keep seeing when teams add another runner backend is that the prompt file starts absorbing lifecycle, memory, and policy concerns that really belong elsewhere.

Keeping runner selection separate from operator policy and evidence plumbing makes debugging much easier later, because you can tell whether a failure came from the tool integration itself or from instruction drift around it. Codex support here feels useful for that reason alone.