Document constrained isolated loader lookup

[pboling]

Summary

• update zero-top-level namespace examples to use an explicit auth-sanitizer requirement
• document a loader.rb presence check before evaluating the isolated loader

Testing

• documentation-only change

[Copilot]

Pull request overview

Updates the README guidance for loading auth-sanitizer without introducing new top-level namespaces, making the loader lookup more explicit and robust.

Changes:

• Update the “Zero Top-Level Namespace Additions” examples to require auth-sanitizer ~> 0.1 and >= 0.1.3.
• Document checking for lib/auth_sanitizer/loader.rb existence and raising a LoadError with a clearer message before attempting to evaluate it.

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[pboling]

This is my own action, so I pin it in my own repos as a canary.