Add IntelliJ IDEA guidance for Java and Kotlin developers

## Summary

Add IntelliJ IDEA-focused Copilot instructions for Java and Kotlin developers.

## Motivation

The repository contains many language- and framework-specific instruction sets, but it does not currently provide guidance tailored to developers working in IntelliJ IDEA. A focused resource would help users combine IntelliJ's project model, inspections, refactorings, debugger, and build-tool integration with portable repository and CI practices.

A related closed question, #668, identified gaps between editor experiences for agents and instructions. This contribution would provide practical guidance for the IntelliJ IDEA audience without depending on IDE-specific project metadata.

## Proposed changes

- Add `instructions/intellij-java-kotlin.instructions.md`.
- Include frontmatter that scopes the instructions to Java, Kotlin, Gradle, Maven, and common IntelliJ project files.
- Cover:
- treating Gradle/Maven configuration and CI as the source of truth;
- using the repository's configured JDK, formatter, inspections, compiler options, and test conventions;
- safe use of IntelliJ refactorings, inspections, navigation, run/debug configurations, and test tools;
- Kotlin null-safety and coroutine conventions where applicable;
- Spring Boot conventions where applicable;
- annotation processing and generated-source handling;
- keeping IDE metadata and generated outputs out of version control unless explicitly required;
- validating changes through the same command-line build and test commands used by CI.
- Add the resource to the appropriate instructions documentation or index so it is discoverable.

## Acceptance criteria

- [ ] An IntelliJ IDEA Java/Kotlin instruction file is added under `instructions/`.
- [ ] The guidance is actionable and IDE-aware without making IDE-only assumptions.
- [ ] It emphasizes portable Gradle/Maven and CI validation.
- [ ] It does not prescribe paid IntelliJ features or require committed IntelliJ metadata.
- [ ] The instructions index or documentation links to the new resource.

Review the existing language- and framework-specific instruction sets and the instructions documentation or index before creating instructions/intellij-java-kotlin.instructions.md. Add the specified IntelliJ-aware, portable Java/Kotlin guidance with scoped frontmatter, then link it from the index and verify every acceptance criterion, including command-line Gradle/Maven and CI validation practices.

Written by the indexing model from the issue text.