Add GitHub Copilot instructions

[Copilot]

Implements repository-specific instructions for GitHub Copilot coding agent as per best practices documentation.

Changes

• Created .github/copilot-instructions.md (216 lines) covering:
  • Julia + C++ + Bazel tech stack conventions
  • Code style: Blue style for Julia, LLVM for C++
  • Project structure: src, ext, lib, deps, test directories
  • Testing strategy: SafeTestsets with 3 test groups (core, integration, neural_networks)
  • Core concepts: ConcreteRArray vs TracedRArray, compilation semantics, XLA backends
  • CI/CD: Multi-platform matrix testing with PJRT/IFRT runtimes
  • Development workflow: formatting, building, common tasks

Provides context for Copilot to understand the tracing-based compilation system, automatic differentiation integration, and multi-backend execution model specific to Reactant.jl.

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.

Additional Instructions

• When ever any changes are made to the repo, make sure to format the files with JuliaFormatter v1 before opening the pull request
• No change should ever be made to the mlir/Dialects directory</issue_description>

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

• Fixes ✨ Set up Copilot instructions #1938

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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.