Add reference to Kotlin code style guide in copilot-instructions.md

[Copilot]

This PR adds a reference to the organization's Kotlin code style guide in the .github/copilot-instructions.md file, following the same pattern implemented in the gradle-common repository.

Changes

Added a new "Code Style" section under the "Code Quality and Validation" heading that points contributors to the Kotlin code style guide.

This ensures consistency across all Kotlin code contributions to the project and aligns with the development guidelines established across the organization's repositories.

Impact

• No functional changes to the codebase
• Documentation-only update to help contributors follow the established Kotlin code style
• Improves consistency with other organization repositories

Original prompt

Do the same as in huanshankeji/gradle-common@1b5a3ef.

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✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[ShreckYe]

Put in a similar section as in kotlin-common. Similar to huanshankeji/kotlin-common#34 (comment).