Add GitHub Copilot instructions for repository

[Copilot]

Configures Copilot coding agent with repository-specific context per best practices guide.

Changes

Added .github/copilot-instructions.md with:

• Flutter plugin context: Multi-platform asset management (Android, iOS, macOS, OpenHarmony) with platform channels
• Code style alignment: Dart/Flutter conventions from analysis_options.yaml (single quotes, trailing commas, const constructors, explicit return types)
• Platform-specific guidelines: Java 17 for Android, Swift/Objective-C for Darwin, OpenHarmony compatibility
• Development workflow: Pre-commit checks (dart format, flutter analyze, flutter test), CHANGELOG.md updates
• Constraints: Minimal changes principle, backward compatibility requirement, no TODO comments in production
• Common patterns: MethodChannel usage, async/await for media operations, platform permissions handling

Guides Copilot to generate contextually appropriate suggestions that match project conventions and multi-platform requirements.

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 #1328

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