fix: lower default permissions for stronghold file

[qrayven]

Description of change

The default permission for the *.stronhold file in the UNIX system is 600

In general, when it comes to storing sensitive content, we should apply the layered security approach. This means, despite the file being encrypted, its default permission should be limited to a minimum to avoid situations when other exploited processes/users can get access to sensitive files.

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Type of change

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• Enhancement (a non-breaking change which adds functionality)

How the change has been tested

manually tested

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Change checklist

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• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes