This PR adds comprehensive GitHub Copilot instructions to help coding agents work effectively in the sakura-editor codebase. The instructions provide essential guidance for working with this Windows-only Japanese text editor project.

Key Features

Windows-Specific Build Requirements: Clear documentation that this is a Windows-only codebase requiring Visual Studio 2017+, Windows 10 SDK, HTML Help Compiler, and Inno Setup.

Validated Build Commands with Timing:

• build-sln.bat - Basic Visual Studio build (2-3 minutes, requires 10+ minute timeout)
• build-all.bat - Complete build including help and installer (5-8 minutes, requires 15+ minute timeout)
• Unit tests with GoogleTest (3-5 minutes, requires 10+ minute timeout)

Critical Development Warnings:

• NEVER CANCEL builds or tests (they may take several minutes)
• Cannot build or run on Linux/macOS (Windows-specific APIs)
• MinGW build exists but has limited functionality
• Manual validation required after making changes

Complete Development Workflow:

• Exact commands for building, testing, and validation
• Project structure explanation (sakura_core/, sakura/, tests/)
• Environment setup including git submodules
• CI validation steps to ensure GitHub Actions will pass

Validation Scenarios: Specific steps to test functionality including running the actual editor, testing Japanese text input, and verifying changes work correctly.

The instructions follow an imperative tone ("Run this command", "Do not do this") and include timing data based on analysis of actual CI builds. They provide everything a coding agent needs to work effectively in this codebase without requiring additional discovery.

Fixes#2131.

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