Harden installer diagnostics for missing prerequisites

[rain2day]

Goal

Make setup failures easier to understand on fresh Macs.

Current Behavior

setup.sh checks for Node, swiftc, Codex.app, and Codex auth, then delegates to install.mjs. The messages are useful but still minimal.

Ideas

• Print detected Node and Swift versions.
• Explain how to install Xcode Command Line Tools when swiftc is missing.
• Add clearer messages for missing Codex auth vs missing Codex.app.
• Keep logs free of tokens or raw credential payloads.

Done When

• A failed setup gives the user one clear next action.
• npm run check still passes.