Initialize JetBrains Junie 🚀

[jetbrains-junie]

This PR initializes JetBrains Junie 🚀 by adding essential configuration files.

Includes:

• GitHub workflow for automated execution
• Dev container setup (if applicable)
• Language-specific scripts (if applicable)

Generated automatically by Junie. Review and customize as needed.

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Summary by cubic

Initialize Junie automation with a reusable GitHub Actions workflow and add a Java 21 dev container. This enables on-demand runs and a consistent dev environment with Maven and Gradle.

• New Features
  • GitHub Actions: adds “Junie” workflow (workflow_dispatch) with inputs run_id and workflow_params; uses jetbrains-junie/junie-workflows/ej-issue.yml; contents: write.
  • Dev container: Java 21 image with Maven 3.8.6 and Gradle preinstalled.

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