Add instruction file for Agents

[BBoyBen]

Pull Request Checklist

• I have read and followed the CONTRIBUTING.md guidelines.
• My contribution adds a new instruction, prompt, or chat mode file in the correct directory.
• The file follows the required naming convention.
• The content is clearly structured and follows the example format.
• I have tested my instructions, prompt, or chat mode with GitHub Copilot.
• I have run npm start and verified that README.md is up to date.

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Description

Adding an instruction file for Copilot agents creation. Describing the agent file configuration :

• Structure / Format
• Tools configuration
• Sub-agent call
• Variable definition
• MCP Configuration

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Type of Contribution

• New instruction file.
• New prompt file.
• New chat mode file.
• New collection file.
• Update to existing instruction, prompt, chat mode, or collection.
• Other (please specify):

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Additional Notes

Instruction principally base on the Github Copilot Agent documentation, and on my tests / projects particullary for the sub-agent / variables

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By submitting this pull request, I confirm that my contribution abides by the Code of Conduct and will be licensed under the MIT License.