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 JetBrains Junie by adding a reusable GitHub Actions workflow and a Java dev container. This enables manual Junie runs and a consistent Java 21 environment with Maven/Gradle.

• New Features
  • GitHub Actions: junie.yml (manual dispatch) calls jetbrains-junie/junie-workflows/ej-issue.yml@main with inputs run_id and workflow_params.
  • Dev Container: Java 21 image with Maven 3.8.6 and Gradle preinstalled for a consistent local setup.

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