Refactor: normalize config values at the config layer

[albertocavalcante]

Summary

Currently, config value normalization (e.g., strings.ToLower for SHA256 hashes) happens at each call site when retrieving values. This leads to repetitive normalization calls scattered throughout the codebase.

Current Pattern

// In getExpectedSHA256
if hash := cfg.Get("BAZELISK_VERIFY_SHA256_" + suffix); hash != "" {
    return strings.ToLower(hash)
}

Similar patterns exist in other places like getBazeliskHome with strings.ToUpper.

Proposed Improvement

Normalize values once at the config layer (when storing/loading), so all consumers can assume values are already in the expected format.

This would:

• Reduce code duplication
• Prevent bugs from forgotten normalization
• Make the codebase cleaner

Related

This was identified while working on #766 (platform-specific SHA256 verification).