Add Copilot instructions for repository

[Copilot]

Configure GitHub Copilot coding agent instructions as documented in best practices guide.

Changes

• Created .github/copilot-instructions.md with repository-specific guidance:
  • Project overview and multi-module Maven structure (9 modules documented)
  • Build environment: Java 11 minimum, Maven commands, test matrix (Java 11/17, GraalVM 17/21)
  • Code guidelines: compilation targets, testing requirements, cross-platform compatibility
  • Task categorization: suitable tasks for Copilot vs. human-required architectural changes
  • Security best practices and documentation standards
  • Links to project resources and communication channels

Context

The instructions align with actual build configuration (maven.compiler.source=11 in pom.xml) and CI matrix (Linux/macOS/Windows on Java 11/17).

Original prompt

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This section details on the original issue you should resolve

<issue_title>✨ Set up Copilot instructions</issue_title>
<issue_description>Configure instructions for this repository as documented in Best practices for Copilot coding agent in your repository.

</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes ✨ Set up Copilot instructions #1176

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[Copilot]

Pull Request Overview

This PR adds GitHub Copilot instructions to guide the coding agent when working on the Nitrite Database repository. The instructions document the project structure, development environment, coding standards, and best practices aligned with the repository's Java 11+ Maven-based build configuration.

Key Changes:

• Created comprehensive Copilot instructions covering project structure, build environment, and coding guidelines
• Documented all 9 Maven modules and their purposes
• Specified Java version requirements (minimum Java 11, testing on Java 11/17, GraalVM 17/21) matching actual build configuration

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