Add Github Copilot modernization for java plugin (#1523)

* sync: java-upgrade plugin update from devdiv-azure-service-dmitryr/azure-java-migration-copilot-vscode-extension@187b2f8

Source commit: 187b2f85130550fbb5213b55347ab4afbb8c17e1

Source branch: xiading/sync-java-2

Triggered by: xiading_microsoft

Workflow: https://github.com/devdiv-azure-service-dmitryr/azure-java-migration-copilot-vscode-extension/actions/runs/24984055440

* Fix modernize-java plugin checklist issues

- Fix keyword casing: 'Java' → 'java' in plugin.json
- Add plugins/modernize-java/README.md based on upstream microsoft/modernize-java
- Remove hook declarations referencing missing scripts (recordToolUse.sh/.ps1)
- Run npm run build to update docs/README.plugins.md, docs/README.agents.md, and marketplace.json

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Fix README install command to use awesome-copilot marketplace

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: app_modernization <app_modernization@example.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.github/plugin/marketplace.json

@@ -345,6 +345,12 @@
         "path": "plugins/modernize-dotnet"
       }
     },
+    {
+      "name": "modernize-java",
+      "source": "modernize-java",
+      "description": "AI-powered Java modernization and upgrade assistant. Helps upgrade Java and Spring Boot applications to the latest versions.",
+      "version": "1.0.0"
+    },
     {
       "name": "napkin",
       "source": "napkin",