[plan] Implement golang.org/x/crypto code quality improvements

[github-actions]

Overview

This tracking issue covers code quality improvements for the golang.org/x/crypto usage in the project, specifically around the NaCl box encryption implementation used for GitHub Actions secrets.

Source: Discussion #6389

Context

The current implementation in pkg/cli/secret_set_command.go is secure and correct ✅. These improvements are focused on code quality and maintainability, not security fixes.

Planned Tasks

This work is broken down into small, focused sub-issues:

1. Add constant for NaCl public key size
2. Document encryption algorithm in function comments
3. Add empty plaintext test case
4. Add round-trip encryption test
5. Document secret size limits in CLI help

Estimated Total Effort

Approximately 45-50 minutes of work across all tasks.

Acceptance Criteria

• All sub-issues are completed and merged
• Code quality is improved with better documentation
• Test coverage includes edge cases
• No breaking changes to existing functionality

AI generated by Plan Command for discussion #6389