Minor Java and README improvements

[ecki]

Pull Request Checklist

• [x] 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.
• [x] 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

Improve Java instruction (maven performance, Java version uptake and Unit test coverqge) Add clarify to instructions README and introduce AGENTS.md

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Type of Contribution

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

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Additional Notes

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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.

[ecki]

Let me know if I should Squash them