Improve compatibility notes for different Codex installs

[sugarpoo]

Document compatibility expectations for different local Codex setups.

Useful areas to cover:

• Codex CLI vs Codex desktop app assumptions
• PATH expectations for LaunchAgent runs
• how to diagnose missing codex commands
• safe fallback workflows when automation is not enabled

Acceptance criteria:

• Update docs without overpromising support.
• Keep guidance beginner-friendly.
• Avoid personal paths, private logs, or machine-specific details.