Add modernize-java plugin

[leonard520]

Added new plugin 'modernize-java' for Java modernization and upgrade assistance.

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, prompt, agent, skill, or workflow 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

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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.

[github-actions]

⚠️ This PR targets main, but PRs should target staged.

The main branch is auto-published from staged and should not receive direct PRs.
Please close this PR and re-open it against the staged branch.

You can change the base branch using the Edit button at the top of this PR,
or run: gh pr edit 1427 --base staged

[Copilot]

Pull request overview

Adds a new external plugin entry for modernize-java so it’s discoverable via the repo’s plugin marketplace data.

Changes:

• Added modernize-java metadata to plugins/external.json.
• Regenerated .github/plugin/marketplace.json to include the new external plugin entry.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
plugins/external.json	Registers modernize-java as an external plugin source (GitHub repo + path) with full metadata.
.github/plugin/marketplace.json	Updates the generated marketplace listing to include modernize-java.