Add .github/copilot-instructions.md for coding agent onboarding

[Copilot]

Adds a copilot-instructions.md to give a coding agent the context needed to work effectively in this repo without manual exploration.

Contents

• Project overview – MOSA's four components (compiler, kernel, device drivers, debugger) and execution model
• Repository layout – Annotated directory tree covering all major project groups (Mosa.Compiler.*, Mosa.Runtime.*, Mosa.BareMetal.*, Mosa.Tool.*, Mosa.Plug.Korlib.*, etc.)
• Build system – Separate solution files for Windows (Mosa.sln) and Linux/macOS (Mosa.Linux.sln); global Directory.Build.props wiring (unified bin/ output, net10.0, C# 14, nullable disabled)
• Testing – Distinction between fast xUnit tests (dotnet test) and MOSA-specific end-to-end tests that compile + run under QEMU, including required QEMU install steps per platform
• CI pipeline – Six-stage matrix across Windows/Linux/macOS × 10 optimization levels (-o0–-o9); NuGet publish gated to master
• Code style – Allman braces, tab indentation, modifier order, var preferences from .editorconfig
• Architecture – Compiler pipeline, plug system, platform backends, bare-metal execution model
• Common pitfalls – Don't set OutputPath per-project, don't enable nullable types, understand Mosa.Linux.sln exclusions, QEMU requirement for integration tests

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