Add privacy-safe source-build troubleshooting for macOS 14+

[Pybsama]

Why

Codex Quota is source-only, so the build instructions are the first experience for new users. The README lists the requirements but does not yet explain the safest way to diagnose common setup failures.

Scope

Add an English troubleshooting subsection to README.md or SUPPORT.md covering:

• an unsupported Swift toolchain;
• Xcode Command Line Tools not being selected;
• an unsupported Intel Mac or macOS version earlier than 14;
• the difference between the ad-hoc development build and a distributable signed app.

Privacy requirements

Use repository-relative or obviously fictional paths only. Do not include complete terminal output, logs, user names, device names, home-directory paths, account information, authentication/state files, or real screenshots.

Acceptance criteria

• The guidance stays source-only and does not claim signing or notarization.
• Commands are minimal and do not read real Codex account data.
• Scripts/verify-public-tree.sh --repository and Scripts/verify-version.sh --tag v0.1.0 pass.
• The documentation remains English.