feat: add comprehensive Java development instructions and best practices

[brijrajsingh]

Pull Request Checklist

• I have read and followed the CONTRIBUTING.md guidelines.
• I have read and followed the Guidance for submissions involving paid services.
• My contribution adds a new instruction 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, agent, skill, or workflow with GitHub Copilot.
• I have run npm start and verified that README.md is up to date.
• I am targeting the staged branch for this pull request.

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Description

This PR adds a Copilot instructions Markdown file for Java code files.

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

[Copilot]

Pull request overview

Adds a new Java-focused Copilot instruction file to guide consistent, modern Java development practices across Java sources and common build files, and registers it in the instructions catalog.

Changes:

• Added instructions/java.instructions.md with comprehensive Java best practices (style, modern features, testing, concurrency, security, build workflow).
• Updated docs/README.instructions.md to include the new Java instructions entry and install links.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
instructions/java.instructions.md	New Java development instruction set covering conventions, modern language features, and engineering practices.
docs/README.instructions.md	Adds a catalog entry pointing to the new Java instruction file.

[aaronpowell]

@brunoborges could I get you to have a review on this, given you've contributed Java items in the past.