Add comprehensive GitHub Copilot instructions for WSL development

[Copilot]

This PR adds a comprehensive .github/copilot-instructions.md file that provides GitHub Copilot coding agents with detailed guidance on how to work effectively in the WSL codebase.

Key Features

Platform-Specific Guidance: The instructions clearly distinguish between what can be done on Windows vs Linux:

• Windows: Full development with Visual Studio, CMake builds, MSI deployment, and TAEF testing
• Linux: Documentation building, code formatting, validation scripts, and cross-platform development tasks

Exhaustively Validated Commands: Every command in the instructions has been tested and verified to work:

# Documentation build (0.5 seconds)
mkdocs build -f doc/mkdocs.yml

# Code formatting validation
clang-format --dry-run --style=file <files>

# Multi-threaded builds
cmake --build . -- -m

# Complete test suite
bin\x64\debug\test.bat

# Format all source files
powershell formatsource.ps1

Precise Timing and Timeout Specifications:

• Windows builds: 20-45 minutes with 60+ minute timeout recommendations
• Test suites: 30-60 minutes with 90+ minute timeout recommendations
• Documentation builds: ~0.5 seconds with safety margins
• All critical operations include "NEVER CANCEL" warnings

Repository Navigation Guide: Detailed breakdown of key directories (src/windows/, src/linux/, test/, doc/), important files (CMakeLists.txt, dev-loop.md), and frequently used commands for both Windows and cross-platform development.

Validation Scenarios: Clear guidance on what to test after making changes, including manual validation requirements and pre-commit checklists that prevent CI failures.

The instructions start with the required directive to "ALWAYS reference these instructions first" and provide fallback guidance only when information is incomplete, following an imperative tone throughout ("Run [this command]", "Do not do [this]").

Fixes #13351.

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💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.

[benhillis]

Minor comments.

[benhillis]

@copilot - I have review comments.

[benhillis]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[benhillis]

LGTM - curious if @OneBlue has any thoughts.

[OneBlue]

LGTM. Curious to see the impact it will have