fix: add GitHub Actions permissions for PR operations

[bartoszclapinski]

• Add explicit permissions to pr-checks.yml workflow

  • contents: read
  • pull-requests: write
  • issues: write

• Add same permissions to dotnet-ci.yml workflow

• Update .github/README.md to clarify it's for workflows only

This fixes the 'Resource not accessible by integration' error that occurred when workflows tried to add labels and comments.

📋 Description

🎯 Type of Change

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📝 Documentation update
• ♻️ Code refactoring
• 🎨 Style/formatting changes
• ✅ Test additions or updates
• ⚙️ Configuration changes

🔗 Related Issue

Closes #(issue number)

📸 Screenshots (if applicable)

✅ Checklist

• My code follows the project's coding standards
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• 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
• Any dependent changes have been merged and published

🧪 Testing

Test Configuration:

• OS: [e.g., Windows 10, Ubuntu 22.04]
• .NET Version: [e.g., 9.0]
• Browser (if applicable): [e.g., Chrome 120]

Test Steps:
1.
2.
3.

📝 Additional Notes

🎓 What I Learned (Optional)

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Note for reviewers: Please check that all CI checks pass before merging.

[bartoszclapinski]

🎯