copilot: Provide more project context for the Copilot coding agent

.github/copilot-instructions.md

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+# GitHub Copilot Instructions for GSL (Guidelines Support Library)
+
+## Project Overview
+This repository contains the Guidelines Support Library (GSL), a Microsoft implementation of types and functions 
+suggested for use by the C++ Core Guidelines. It's a header-only C++ library with emphasis on safety, 
+correctness, and zero overhead.
+
+## Coding Standards
+
+### General
+- Follow C++ Core Guidelines wherever possible
+- Use meaningful type, function, and template parameter names
+- Keep functions small and focused with clear preconditions/postconditions
+- Include comments for complex code, but prefer self-documenting code
+- Use the Expects() and Ensures() macros for contract verification
+
+### Style Guidelines
+- Use 4 spaces for indentation (not tabs)
+- Maximum line length of 100 characters
+- Follow GSL naming conventions (lowercase with underscores)
+- Keep templates clean and readable with appropriate spacing
+- Use C++14 features since this is the minimum standard supported
+
+### Error Handling
+- Use Expects() for preconditions and Ensures() for postconditions
+- Design for fail-fast semantics (std::terminate) on contract violations
+- Template constraints should use static_assert or SFINAE
+- Don't throw exceptions from basic operations
+
+### Testing
+- Write thorough unit tests for every component using GTest
+- Test for all edge cases and error conditions
+- Ensure cross-platform compatibility in tests
+- Maintain 100% code coverage for changed code
+
+## Project-Specific Conventions
+
+### Architecture
+- All public types must be in the gsl namespace
+- Design for zero overhead abstractions when possible
+- Respect the distinction between Owners and Views
+- Maintain backward compatibility with existing GSL code
+
+### Version Control
+- Link all PRs to related issues
+- Use clear commit messages explaining what and why
+- Follow the contribution guidelines documented in CONTRIBUTING.md
+- PRs should include appropriate tests with 100% coverage for changed code
+
+### Documentation
+- Document all public APIs with clarity on preconditions and postconditions
+- Keep header comments up-to-date
+- Include examples for complex functionality in docs/headers.md
+
+## Technology Stack
+- C++14 (minimum) for core implementation
+- CMake build system (3.14+)
+- Google Test for unit testing
+- Support for multiple compilers (MSVC, GCC, Clang)
+
+## Security Considerations
+- Bounds checking is a core principle - enforce it consistently
+- Design for safety while minimizing overhead
+- Ensure undefined behavior is explicitly detected where possible
+
+## Performance Guidelines
+- Optimize for both safety and performance
+- Constexpr-enable functions wherever possible
+- Avoid hidden allocations
+- Use noexcept appropriately for move operations and other performance-critical functions
+
+## Cross-Platform Support
+- Code must work across:
+  - Windows (MSVC)
+  - Linux (GCC, Clang)
+  - macOS (AppleClang)
+  - Android and iOS where applicable
+
+## Copilot Tasks
+- You can find the CMake artifacts for C++20 in build-cxx20 and C++14 in build-cxx14.
+- Before publishing a PR, verify the following:
+  - There are no compiler warnings or errors when building the test suite.
+  - The test suite passes on all supported platforms and compilers.
+  - The test suite passes for both C++14 and C++20.

.github/workflows/copilot-setup-steps.yml

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+name: "Copilot Setup Steps"
+
+on: workflow_dispatch
+
+jobs:
+  copilot-setup-steps:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Install Build Dependencies
+        run: sudo apt-get update && sudo apt-get install -y clang cmake make
+
+      - name: Configure CMake (C++14)
+        run: cmake -B build-cxx14 . -DGSL_CXX_STANDARD=14 -DGSL_TEST=ON -G "Unix Makefiles"
+
+      - name: Configure CMake (C++20)
+        run: cmake -B build-cxx20 . -DGSL_CXX_STANDARD=20 -DGSL_TEST=ON -G "Unix Makefiles"
+