Add GitHub Copilot setup steps workflow for development environment

[Copilot]

This PR implements a GitHub Copilot setup steps workflow to customize the development environment for the Copilot coding agent, following the official GitHub documentation.

What this enables

GitHub Copilot coding agent will now have access to a fully configured development environment that mirrors our CI setup, allowing it to:

• Build and compile Dash Core from source
• Run unit tests and functional tests
• Execute linters and code quality tools
• Make meaningful code changes with proper validation

Implementation details

The .github/workflows/copilot-setup-steps.yml workflow:

Environment Setup:

• Ubuntu with all required system packages (build-essential, autotools, cmake)
• LLVM/Clang 18 toolchain with development tools (clang-format, clang-tidy)
• Python 3.9.18 matching .python-version with required packages (codespell, flake8, mypy)
• Additional tools (shellcheck, cppcheck, ccache) used by our linting and build processes

Build Process:

• Generates build system with ./autogen.sh
• Builds dependencies using make -C depends for the native platform
• Configures project with development flags and clang compiler
• Builds Dash Core with parallel compilation

Performance Optimizations:

• Multi-layer caching for apt packages, depends sources/builds, and ccache
• Parallel builds using all available CPU cores
• Smart cache keys based on file hashes to minimize rebuild times
• 59-minute timeout respecting GitHub's limits

Validation:

• Runs basic unit tests to verify the build works correctly
• Tests dashd startup/shutdown functionality
• Verifies all linting tools are available and functional

Why this matters

Without this setup, Copilot would need to discover and install dependencies through trial and error, which is slow and unreliable. This pre-configured environment ensures Copilot can immediately start working with the codebase effectively, understanding build failures, running tests, and making validated code changes.

The workflow automatically triggers on changes to validate the setup and can be manually run from the Actions tab for testing.

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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.