feat: add .NET timezone handling guidance and reference

[pasindudilshan1]

Pull Request Checklist

• I have read and followed the CONTRIBUTING.md guidelines.
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Description

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Type of Contribution

• New instruction file.
• New prompt file.
• New agent file.
• New plugin.
• New skill file.
• New agentic workflow.
• Update to existing instruction, prompt, agent, plugin, skill, or workflow.
• Other (please specify):

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Additional Notes

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@all-contributors
please add @pasindudilshan1 for skills.

[aaronpowell]

closing as duplicate of #1123